[analyzer] Cache the "concrete offset base" for regions with symbolic offsets.

This makes it faster to access and invalidate bindings with symbolic offsets
by only computing this information once.

No intended functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161635 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed