c++: ICE in is_really_empty_class [PR95497]

c++: ICE in is_really_empty_class [PR95497]

We are ICEing in the testcase below because we pass the
yet-uninstantiated class type A<int> of the PARM_DECL b to
is_really_empty_class from is_rvalue_constant_expression when parsing
the requirement t += b.

This patch fixes the ICE by guarding the problematic call to
is_really_empty_class with a COMPLETE_TYPE_P check, which should also
subsume the existing dependent_type_p check.

gcc/cp/ChangeLog:

	PR c++/95497
	* constexpr.c (potential_constant_expression_1) <case PARM_DECL>:
	When processing_template_decl, check COMPLETE_TYPE_P before
	calling is_really_empty_class.  Don't check dependent_type_p.

gcc/testsuite/ChangeLog:

	PR c++/95497
	* g++.dg/cpp2a/concepts-pr95497.C: New test.