Remove lval::FieldOffset, lval::ArrayOffset. These will be replaced with regions.

Remove lval::FieldOffset, lval::ArrayOffset. These will be replaced with regions.
Remove GRExprEngine::getLVal and RValues::MakeVal.
Enhance StoreManager "GetLValue" methods to dispatch for specific kinds of lvalue queries, as opposed to interogating the expression tree (GRExprEngine already does this).

Added FIXMEs.  In particular, we no longer "assume" that a base pointer in a field/array access is null (this logic was removed).  Perhaps we should do this when fetching the lvalue for fields and array elements?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57657 91177308-0d34-0410-b5e6-96231b3b80d8