Preprocessor: Add __ALIGNOF_MAX_ALIGN_T__

TargetInfo::getSuitableAlign() was introduced in r146762 and is defined
as alignof(std::max_align_t).

Introduce __ALIGNOF_MAX_ALIGN_T__ which exposes getSuitableAlign() so
that libc++ may take advantage of it.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201037 91177308-0d34-0410-b5e6-96231b3b80d8