[analyzer] Refactor RegionStore's sub-region bindings traversal.

This is going to be used in the next commit.
While I'm here, tighten up assumptions about symbolic offset
BindingKeys, and make offset calculation explicitly handle all
MemRegion kinds.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175228 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed