[CFLAA] Add offset tracking in CFLGraph.

(Also, refactor our constexpr handling to be less insane).

This patch lets us track field offsets in the CFL Graph, which is the
first step to making CFLAA field/offset sensitive. Woohoo! Note that
this patch shouldn't visibly change our behavior (since we make no use
of the offsets we're now tracking), so we can't quite add tests for this
yet.

Patch by Jia Chen.

Differential Revision: https://reviews.llvm.org/D22598

llvm-svn: 276201