[Metadata] Add TBAA struct metadata to `AAMDNode`

Summary:
Make `AAMDNodes`' `getAAMetadata()` and `setAAMetadata()` to take `!tbaa.struct`
into account as well as `!tbaa`. This impacts llvm.org/pr42022.
This is a temprorary fix needed to keep `!tbaa.struct` tag by SROA pass.
New field `TBAAStruct` should be deleted when `!tbaa` tag replaces `!tbaa.struct`.
Merging two `!tbaa.struct`'s to one is conservatively considered to be `nullptr`
(giving `MayAlias`) -- this could be enhanced, but relying on the said future
replacement.

Reviewers: RKSimon, spatel, vporpo

Subscribers: hiraditya, kosarev, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70924