[CodeGen] Fix propagation of TBAA info for atomic accesses

This patch fixes clang to propagate complete TBAA information for
atomic accesses and not just the final access types. Prepared
against D38456 and requires it to be committed first.

This is part of D37826 reworked to be a separate patch to
simplify review.

Differential Revision: https://reviews.llvm.org/D38460

llvm-svn: 314784