[clang] ICE on handling reference to consteval function in initializer

[groundswellaudio]

namespace ns
{
    consteval int foo(int x) { return 1; }
}

template <class A>
struct T {
    static constexpr auto xx = ns::foo(A{});
};

crash with

Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++20 <source> -isystem/opt/compiler-explorer/libs/catch2/v3.0.1/src
1.	<source>:9:44: current parser token ';'
2.	<source>:8:1: parsing struct/union/class body 'T'
 #0 0x00000000033cba18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x33cba18)
 #1 0x00000000033c9b44 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x33c9b44)
 #2 0x0000000003318b68 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007fc4f42c7420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #4 0x0000000000ad8e28 clang::Sema::MarkExpressionAsImmediateEscalating(clang::Expr*) (.cold) SemaExpr.cpp:0:0
 #5 0x0000000005f39f3d clang::Sema::PopExpressionEvaluationContext() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5f39f3d)
 #6 0x0000000005a025e4 clang::Parser::ParseCXXMemberInitializer(clang::Decl*, bool, clang::SourceLocation&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a025e4)
 #7 0x0000000005a15a5f clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo const&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a15a5f)
 #8 0x0000000005a16d34 clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&, clang::ParsedAttributes&, clang::TypeSpecifierType, clang::Decl*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a16d34)
 #9 0x0000000005a173a6 clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a173a6)
#10 0x0000000005a19679 clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo const&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a19679)
#11 0x00000000059e9414 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo const&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x59e9414)
....

https://godbolt.org/z/5a8hPo5je

On Clang 16, this used to error out with :

<source>:9:32: error: cannot take address of consteval function 'foo' outside of an immediate invocation
    static constexpr auto xx = ns::foo(A{});
                               ^
<source>:4:19: note: declared here
    consteval int foo(int x) { return 1; }

I think the problem stems from
1/ during the parsing of the initializer, the ExpressionEvaluationContext is not set to ConstantEvaluated, so the reference to ns::foo is added to ReferenceToConsteval
2/ When popping the Sema::PopExpressionEvalContext is called, Context.InImmediateEscalatingFunctionContext is true, most likely because ParseCXXMemberInitializer contains this line :

 Actions.ExprEvalContexts.back().InImmediateEscalatingFunctionContext = true;

Which does not makes a lot of sense to me (besides this kind of state mutation would be more suitable on the Sema side? e.g. in Sema::ActOnStartCXXInClassMemberInitializer/Finish).

Finally, the evaluation context should probably be set to ConstantEvaluated when the variable is constexpr to not error on the declaration reference.

[shafik]

CC @cor3ntin

[llvmbot]

@llvm/issue-subscribers-clang-frontend

```cpp namespace ns { consteval int foo(int x) { return 1; } }

template <class A>
struct T {
static constexpr auto xx = ns::foo(A{});
};

crash with 

Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++20 <source> -isystem/opt/compiler-explorer/libs/catch2/v3.0.1/src

1. <source>:9:44: current parser token ';'
2. <source>:8:1: parsing struct/union/class body 'T'
   #0 0x00000000033cba18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x33cba18)
   #1 0x00000000033c9b44 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x33c9b44)
   #2 0x0000000003318b68 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
   #3 0x00007fc4f42c7420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
   #4 0x0000000000ad8e28 clang::Sema::MarkExpressionAsImmediateEscalating(clang::Expr*) (.cold) SemaExpr.cpp:0:0
   #5 0x0000000005f39f3d clang::Sema::PopExpressionEvaluationContext() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5f39f3d)
   #6 0x0000000005a025e4 clang::Parser::ParseCXXMemberInitializer(clang::Decl*, bool, clang::SourceLocation&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a025e4)
   #7 0x0000000005a15a5f clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo const&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a15a5f)
   #8 0x0000000005a16d34 clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&, clang::ParsedAttributes&, clang::TypeSpecifierType, clang::Decl*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a16d34)
   #9 0x0000000005a173a6 clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a173a6)
   #10 0x0000000005a19679 clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo const&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5a19679)
   #11 0x00000000059e9414 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo const&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x59e9414)
   ....

https://godbolt.org/z/5a8hPo5je

On Clang 16, this used to error out with : 

<source>:9:32: error: cannot take address of consteval function 'foo' outside of an immediate invocation
static constexpr auto xx = ns::foo(A{});
^
<source>:4:19: note: declared here
consteval int foo(int x) { return 1; }

I think the problem stems from 
1/ during the parsing of the initializer, the ExpressionEvaluationContext is not set to ConstantEvaluated, so the reference to ns::foo is added to ReferenceToConsteval
2/ When popping the Sema::PopExpressionEvalContext is called, `Context.InImmediateEscalatingFunctionContext` is true, most likely because `ParseCXXMemberInitializer` contains this line : 

Actions.ExprEvalContexts.back().InImmediateEscalatingFunctionContext = true;

Which does not makes a lot of sense to me (besides this kind of state mutation would be more suitable on the Sema side? e.g. in Sema::ActOnStartCXXInClassMemberInitializer/Finish).

Finally, the evaluation context should probably be set to ConstantEvaluated when the variable is `constexpr` to not error on the declaration reference. 
</details>

[cor3ntin]

@shafik this is #66021