[TBAA] Handle bitfields when generating !tbaa.struct metadata. (#82922)

At the moment, clang generates what I believe are incorrect !tbaa.struct
fields for named bitfields. At the moment, the base type size is used
for named bifields (e.g. sizeof(int)) instead of the bifield width per
field. This results in overalpping fields in !tbaa.struct metadata.

This causes incorrect results when extracting individual copied fields
from !tbaa.struct as in added in dc85719d.

This patch fixes that by skipping by combining adjacent bitfields
in fields with correct sizes.

Fixes https://github.com/llvm/llvm-project/issues/82586