[NFC] Remove LLVM_ALIGNAS

Summary: The minimum compilers support all have alignas, and we don't use LLVM_ALIGNAS anywhere anymore. This also removes an MSVC diagnostic which, according to the comment above, isn't relevant anymore.

Reviewers: rnk

Subscribers: mgorny, jkorous, dexonsmith, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D65458

llvm-svn: 367383