c++: Partially revert fix for PR c++/95497 [PR96132]

c++: Partially revert fix for PR c++/95497 [PR96132]

I was mistaken to assume that a dependent type is necessarily
incomplete, and indeed there are multiple places in the frontend where
we check a type for both dependency and completeness.  So this patch
partially reverts the fix for PR95497, restoring the dependent_type_p
check that guarded the call to is_really_empty_class below.

gcc/cp/ChangeLog:

	PR c++/96132
	* constexpr.c (potential_constant_expression_1) <case PARM_DECL>:
	Restore dependent_type_p check that guarded the call to
	is_really_empty_class.

gcc/testsuite/ChangeLog:

	PR c++/96132
	* g++.dg/template/incomplete12.C: New test.