[TBAA] Test for tbaa.struct metadata with bitfields in big endian layout (#87617)

This test exposes what I think is invalid tbaa.struct metadata currently
generated for bitfields when using big endian layout. The regions given
by `!{i64 2, i64 4, [[META3:![0-9]+]], i64 4, i64 4 ...` are
overlapping. This issue was originally observed in
https://github.com/llvm/llvm-project/pull/86709.