[clang][dataflow] Add support for return values of reference type.

This patch changes the way `Environment::ReturnLoc` is set: Whereas previously it was set by the caller, it is now set by the callee (obviously, as we otherwise would not be able to return references).

The patch also introduces `Environment::ReturnVal`, which is used for non-reference-type return values. This allows these to be handled with the correct value category semantics; see also https://discourse.llvm.org/t/70086, which describes the ongoing migration to strict value category semantics.

Depends On D150776

Reviewed By: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D151194