[CodeGen] Do not refer to complete TBAA info where we actually deal with just TBAA access types

This patch fixes misleading names of entities related to getting,
setting and generation of TBAA access type descriptors.

This is effectively an attempt to provide a review for D37826 by
breaking it into smaller pieces.

Differential Revision: https://reviews.llvm.org/D38404

llvm-svn: 314657