Teach libc++ to use the compiler-provided C-compatible ::max_align_t
rather than its own type for std::max_align_t. This is particularly
relevant as the types may not be ABI compatible despite users expecting
them to be.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@201843 91177308-0d34-0410-b5e6-96231b3b80d8