Makes GRState::makeWithStore private, to encourage clients to make store changes...

Makes GRState::makeWithStore private, to encourage clients to make store changes through GRState instead of directly accessing the StoreManager. Also adds cover methods for InvalidateRegion(s) and EnterStackFrame to GRState.

This is in preparation for proposed region change notifications. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110137 91177308-0d34-0410-b5e6-96231b3b80d8