[clang] [Clang] Fix missing diagnostic for non-standard layout type in `offsetof` (PR #65246)

[Aaron Ballman via cfe-commits]

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

Hmmm, in some ways I think this is making the diagnostic needlessly chatty, and in other ways the new diagnostic behavior seems reasonable.

On balance, I think the improvements outweigh the regressions, especially for code users are likely to hit. e.g., I suspect there's way more use of offsetof in a static assertion than there are uses of offsetof as an operand to sizeof.

https://github.com/llvm/llvm-project/pull/65246