Avoid using std::max_align_t in pre-C++11 mode
Always depend on the compiler to have a correct implementation of
max_align_t in stddef.h and don't provide a fallback. For pre-C++11,
require STDCPP_NEW_ALIGNMENT in <new> as provided by clang in all
standard modes. Adjust test cases to avoid testing or using max_align_t
in pre-C++11 mode and also to better deal with alignof(max_align_t)>16.
Document requirements of the alignment tests around natural alignment of
Differential revision: https://reviews.llvm.org/D73245