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

llvm-svn: 300914