re PR debug/12103 (-g only crash (segfault) with a forward declared class used as a template parameter)

	PR debug/12103
	* class.c (update_vtable_entry_for_fn): Do not go through
	covariance machinery if the type returned by an overrider is the
	same as the original.

	PR debug/12103
	* g++.dg/debug/crash1.C: New test.

From-SVN: r78680