Unconditionally lower std::string's alignment requirement from 16 to 8.

As requested in llvm#68807

libcxx/docs/ReleaseNotes/18.rst

@@ -133,6 +133,12 @@ ABI Affecting Changes
   results in an ABI break, however in practice we expect uses of ``std::projected`` in ABI-sensitive places to be
   extremely rare. Any error resulting from this change should result in a link-time error.
 
+- The internal alignment requirements for heap allocations inside std::string has decreased from 16 to 8.
+  This save memory since string requests fewer additional bytes than it did previously. However, this
+  also changes the return value of std::string::max_length and can cause code compiled against older
+  libc++ versions but linked at runtime to a new version to thrown a different exception
+  when attempting allocations that are too large (std::bad_alloc vs std::length_error).
+
 Build System Changes
 --------------------
 

libcxx/include/string

@@ -1851,7 +1851,7 @@ private:
         _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
         size_type __align_it(size_type __s) _NOEXCEPT
             {return (__s + (__a-1)) & ~(__a-1);}
-    enum {__alignment = 16};
+    enum { __alignment = 8 };
     static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     size_type __recommend(size_type __s) _NOEXCEPT
     {

libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// This test demonstrates the smaller allocation sizes when the alignment
+// requirements of std::string are dropped from 16 to 8.
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <string>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  std::string input_string;
+  input_string.resize(64, 'a');
+
+  // Call a constructor which selects its size using __recommend.
+  std::string test_string(input_string.data());
+  constexpr std::size_t expected_align8_size = 71;
+  // Previously, when the alignment used to be 16 bytes, the expected
+  // capacity was 79.
+  assert(test_string.capacity() == expected_align8_size);
+}

libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp

@@ -18,7 +18,8 @@
 #include "test_macros.h"
 
 // alignment of the string heap buffer is hardcoded to 16
-static const std::size_t alignment = 16;
+
+static const std::size_t alignment = 8;
 
 template <class = int>
 TEST_CONSTEXPR_CXX20 void full_size() {