[clang][dataflow] Fix bug in handling of `return` statements.

The handling of return statements, added in support of context-sensitive
analysis, has a bug relating to functions that return reference
types. Specifically, interpretation of such functions can result in a crash from
a bad cast. This patch fixes the bug and guards all of that code with the
context-sensitive option, since there's no reason to execute at all when
context-sensitive analysis is off.

Differential Revision: https://reviews.llvm.org/D140430