[CodeGen] Refine generation of TBAA info for bit-field lvalues

The main change is that now we generate TBAA info before
constructing the resulting lvalue instead of constructing lvalue
with some default TBAA info and fixing it as necessary
afterwards. We also keep the TBAA info close to lvalue base info,
which is supposed to simplify their future merging.

This patch should not bring in any functional changes.

This is part of D38126 reworked to be a separate patch to
simplify review.

Differential Revision: https://reviews.llvm.org/D38947

llvm-svn: 315989