[LangRef] Update the TBAA section

Summary:
Update the TBAA section to mention the struct path TBAA that LLVM
implements today.  This is not a proposal or change in semantics -- it
is intended only to **document** what LLVM already does today.

This is related to https://reviews.llvm.org/D26438 where I've tried to
implement some of the constraints as verifier checks.

Reviewers: anna, reames, rsmith, chandlerc, hfinkel, rjmccall, mehdi_amini, dexonsmith, manmanren

Reviewed By: manmanren

Subscribers: dberlin, dberris, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D26831

llvm-svn: 294999