[analyzer] Mark the root SVal class as isPodLike.

Pure optimization, no functionality change. Probably does not make much
of a difference, but it's free.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176229 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] RegionStore: collectSubRegionKeys -> collectSubRegionBindings

By returning the (key, value) binding pairs, we save lookups afterwards.
This also enables further work later on.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176230 91177308-0d34-0410-b5e6-96231b3b80d8