Initial work on splitting the ValueState into an Environment, Store, and

Constraints. These concepts are already present in the current ValueState, but
the implementation is monolothic. Making ValueState more modular opens up new
design choices for customizing the analysis engine.

In the context of the analysis engine, the "Environment" is the binding between
Expr* (expressions) and intermediate symbolic values (RValues).

llvm-svn: 53252