[clang][dataflow] Eliminate `ReferenceValue`.

There are no remaining uses of this class in the framework.

This patch is part of the ongoing migration to strict handling of value categories (see https://discourse.llvm.org/t/70086 for details).

Reviewed By: ymandel, xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D155922