Fixes https://github.com/llvm/llvm-project/issues/58637.
Adds `isAlignas()` method on `AttributeCommonInfo` which accounts for
C++ `alignas` as well as C11 `_Alignas`.
The method is used to improve diagnostic in C when `_Alignas` is used in
C at the wrong location. This corrects the previously suggested move
of `_Alignas` past the declaration specifier, now warns attribute
`_Alignas` is ignored.
Based on https://reviews.llvm.org/D141177.
2e7913342e