[clang] [C++17] Support __GCC_[CON|DE]STRUCTIVE_SIZE (PR #89446)

[Aaron Ballman via cfe-commits]

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@@ -89,6 +89,19 @@ sections with improvements to Clang's support for those languages.
 C++ Language Changes
 --------------------
 
+C++17 Feature Support
+^^^^^^^^^^^^^^^^^^^^^
+- Clang now exposes ``__GCC_DESTRUCTIVE_SIZE`` and ``__GCC_CONSTRUCTIVE_SIZE``
+  predefined macros to support standard library implementations of
+  ``std::hardware_destructive_interference_size`` and
+  ``std::hardware_constructive_interference_size``, respectively. These macros
+  are predefined in all C and C++ language modes. These macros can be
+  overridden on the command line with ``-D``, if desired. The values the macros
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AaronBallman wrote:

Hmm, given that I'm not implementing the on-by-default diagnostic at this time, it's probably reasonable to remove that bit. Thanks!

https://github.com/llvm/llvm-project/pull/89446