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

At the moment,distinct fields for each union member are generated. When
copying a union, we don't know which union member is active, so there's
no benefit from recording the different fields. It can result in
converting tbaa.struct fields to incorrect tbaa nodes when extracting
fields.

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