For GRExprEngine::EvalBind() (and called visitors), unifiy StoreE and AssignE....

For GRExprEngine::EvalBind() (and called visitors), unifiy StoreE and AssignE.  Now StoreE (const Stmt*) represents the expression where the store took place, which is the assignment expression if it takes place in an assignment.  This removes some conceptual dissidence as well as removes an extra parameter from the Checker::PreVisitBind() visitor.  It also improves ranges and source location information in analyzer diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@112789 91177308-0d34-0410-b5e6-96231b3b80d8