Localize the special processing of array variable inside

Localize the special processing of array variable inside
GRExprEngine::VisitCast() so that other parts of the analyzer can be ignorant.
When we cast "array of type T" to "pointer to T", we get the loc::MemRegionVal
corresponding to the array variable. This is sufficient for BasicStore, but not
for RegionStore. RegionStore should get the element region for the first array
element in the cast. So next comes to the revamping of transfer functions for
different store model.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57897 91177308-0d34-0410-b5e6-96231b3b80d8