Extract a TBAAVerifier out of the verifier (NFC)

This is intended to be used (in a later patch) by the BitcodeReader
to detect invalid TBAA and drop them when loading bitcode, so that
we don't break client that have legacy bitcode with possible invalid
TBAA.

Differential Revision: https://reviews.llvm.org/D27838

llvm-svn: 289927