[IR] Add MDBuilder helpers for the new TBAA metadata format

The new helpers are supposed to be used in clang to generate TBAA
information in the new format proposed in this thread:

http://lists.llvm.org/pipermail/llvm-dev/2017-November/118748.html

Differential Revision: https://reviews.llvm.org/D39956

llvm-svn: 320993