Add MDBuilder utilities for path-aware TBAA.

Add utilities to create struct nodes in TBAA type DAG and to create path-aware
tags. The format of struct nodes in TBAA type DAG: a unique name, a list of
fields with field offsets and field types. The format of path-aware tags:
a base type in TBAA type DAG, an access type and an offset relative to the base
type.

llvm-svn: 178564