Headers: Provide an ABI compatible max_align_t when _MSC_VER is defined

Summary:
Our usual definition of max_align_t wouldn't match up with MSVC if it
was used in a template argument.

Reviewers: chandlerc, rsmith, rnk

Reviewed By: chandlerc

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2924

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202911 91177308-0d34-0410-b5e6-96231b3b80d8