[clang] [clang][CodeGen] Fix crash on non-natural type in CheckAtomicAlignment (PR #141053)

[Matt Arsenault via cfe-commits]

https://github.com/arsenm approved this pull request.

Doesn't seem right that this is looking to the IR for the size instead of directly at the source type 

https://github.com/llvm/llvm-project/pull/141053