[CodeGen] Introduce generic TBAA access descriptors

[CodeGen] Introduce generic TBAA access descriptors

With this patch we implement a concept of TBAA access descriptors
that are capable of representing both scalar and struct-path
accesses in a generic way.

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

Differential Revision: https://reviews.llvm.org/D38456

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314780 91177308-0d34-0410-b5e6-96231b3b80d8