This is the first step to build a better evaluation model for GRExprEngine. A

This is the first step to build a better evaluation model for GRExprEngine.  A
new VisitLValue method is added to replace the old VisitLVal. The semantics
model becomes more explicit to separate rvalue evaluation from lvalue
evaluation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57627 91177308-0d34-0410-b5e6-96231b3b80d8