[clang] [C++] Fix parsing of _Alignas in local declarations (PR #81915)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

CI came back green (failures are unrelated to this patch) and the changes are trivial, so landing as a code owner (I'll happily address any feedback post-commit).

https://github.com/llvm/llvm-project/pull/81915