c++: ICE with concept, local class, and lambda [PR115561]

c++: ICE with concept, local class, and lambda [PR115561]

Here when we want to synthesize methods for foo()::B maybe_push_to_top_level
calls push_function_context, which sets cfun to a dummy value; later
finish_call_expr tries to set something in
cp_function_chain (i.e. cfun->language), which isn't set.  Many places in
the compiler check cfun && cp_function_chain to avoid this problem; here we
also want to check !cp_unevaluated_operand, like set_flags_from_callee does.

	PR c++/115561

gcc/cp/ChangeLog:

	* semantics.cc (finish_call_expr): Check cp_unevaluated_operand.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda21.C: New test.