[clang][dataflow] Handle return statements

This patch adds a `ReturnLoc` field to the `Environment`, serving a similar to the `ThisPointeeLoc` field in the `DataflowAnalysisContext`. It then uses that (along with a new `VisitReturnStmt` method in `TransferVisitor`) to handle non-`void`-returning functions in context-sensitive analysis.

Reviewed By: ymandel, sgatev

Differential Revision: https://reviews.llvm.org/D130600