[clang] [TBAA] Generate tbaa.struct single field with char tag for unions. (PR #84370)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

This seems fine.

In some cases, we could maybe prove certain bytes are padding regardless of the current representation, but probably not worth the effort to implement.

https://github.com/llvm/llvm-project/pull/84370