[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367383 91177308-0d34-0410-b5e6-96231b3b80d8

docs/doxygen.cfg.in

@@ -1925,7 +1925,7 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             = LLVM_ALIGNAS(x)=
+PREDEFINED             =
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The