Add __CLANG_ATOMIC_<TYPE>_LOCK_FREE macros for use in MSVC compatibility mode.
Summary:
Libc++ currently implements the `ATOMIC_<TYPE>_LOCK_FREE` macros using the `__GCC_ATOMIC_<TYPE>_LOCK_FREE` macros. However these are not available when MSVC compatibility is enabled even though C11 `_Atomic` is. This prevents libc++ from correctly implementing `ATOMIC_<TYPE>_LOCK_FREE`.
This patch adds an alternative spelling `__CLANG_ATOMIC_<TYPE>_LOCK_FREE` that is enabled with `-fms-compatibility`.
Reviewers: rsmith, aaron.ballman, majnemer, zturner, compnerd, jfb, rnk
Reviewed By: rsmith
Subscribers: BillyONeal, smeenai, jfb, cfe-commits, dschuff
Differential Revision: https://reviews.llvm.org/D32265
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300914
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