[CFLAA] Add more offset-sensitivity tracking.

This patch teaches FunctionInfo about offsets.

Like the last patch, this one doesn't introduce any visible
functionality change (the core algorithm knows nothing about offsets;
they're just plumbed through). Tests will come when we start acting
differently because of the offsets.

Patch by Jia Chen.

(N.B. I made a tiny change to Jia's patch to avoid warnings by GCC: I
put DenseMapInfo specializations in the `llvm` namespace. Only realized
that those appeared when compiling locally. :) )

Differential Revision: https://reviews.llvm.org/D22634

llvm-svn: 276486