Fix ATOMIC_<TYPE>_LOCK_FREE macros on Windows.

Previously the ATOMIC_<TYPE>_LOCK_FREE macros were implemented
using __GCC_ATOMIC_<TYPE>_LOCK_FREE but GCC specific macros
are defined when -fms-compatibility is specified.

To avoid this Libc++ now tries to use the newly added
__CLANG_ATOMIC_<TYPE>_LOCK_FREE macros instead, and only falls
back to the GCC versions when the Clang ones aren't available.

llvm-svn: 300920