Introduces a new BindingVal which combines direct and

default binding for regions. This allows us to simply a lot of code. A
further simplification could be done is that many methods of
regionstore can only work on Store instead of GRState. 


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@83762 91177308-0d34-0410-b5e6-96231b3b80d8