Fix compile error: Use explicit conversion of unique_ptr to bool

The aging LLVM code is converting an object of type `unique_ptr` to an
object of type `bool`, but is doing so implicitly, thereby causing a
compile-time error with modern, standards-compliant compilers.

This is explained well here:

    microsoft/DirectXShaderCompiler#1692

In particular:

> The C++ standard specifies operator bool on unique_ptr
> as being explicit, so this is clearly a bug in the LLVM
> source code: https://en.cppreference.com/w/cpp/memory/unique_ptr/operator_bool

This commit merely makes the conversion explicit, allowing
the code to be built by GCC version 9.1.0.

This bug was fixed by the LLVM project proper as part of
the following commit (more than 4 years ago):

    69341e6abca92f7f118ee7bd99be0cdfc649386f
    llvm/llvm-project@69341e6

include/llvm/IR/ValueMap.h

@@ -101,7 +101,7 @@ class ValueMap {
 
   ~ValueMap() {}
 
-  bool hasMD() const { return MDMap; }
+  bool hasMD() const { return bool(MDMap); }
   MDMapT &MD() {
     if (!MDMap)
       MDMap.reset(new MDMapT);