[clang][PR55406] CFG for coroutine

CoreturnStmt needs to keep the operand value distinct from its use in
any return_value call, so that instantiation may rebuild the latter.
But it also needs to keep the operand value separate in the case of
calling return_void.  Code generation checks the operand value form to
determine whether it is a distincte entity to the promise call.  This
adds the same logic to CFG generation.

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D126399