Fix TBAA for unsigned fixed-point types

Summary:
Unsigned types can alias the corresponding signed types. I don't see
that this is explicitly mentioned in the Embedded-C specification, but
I think it should work the same as for the integer types.

Patch by: materi

Reviewers: ebevhan, leonardchan

Reviewed By: leonardchan

Subscribers: kosarev, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76856