Restore the libc++ definition of max_align_t on Apple platforms

Clang has introduced ::max_align_t in stddef.h in r201729, but libc++ was
already defining std::max_align_t on Darwin because there was none in the
global namespace.  After that Clang commit though, libc++ started defining
std::max_align_t to be a typedef for ::max_align_t, which has a different
definition.  This changed the ABI.  This commit restores the previous
definition.

rdar://19919394 rdar://18557982

llvm-svn: 230292