c++: Fix FE devirt with diamond inheritance [PR95158]

c++: Fix FE devirt with diamond inheritance [PR95158]

This started breaking in GCC 8 because of the fix for PR15272; after that
change, we (correctly) remember the lookup from template parsing time that
found Base::foo through the non-dependent MiddleB base, and so we overlook
the overrider in MiddleA.  But given that, the devirtualization condition
from the fix for PR59031 is insufficient; we know that d has to be a
Derived, and we found Base::foo in Base, but forcing a non-virtual call
gets the wrong function.

Fixed by removing the PR59031 code that the PR67184 patch moved to
build_over_call, and instead looking up the overrider in BINFO_VIRTUALS.

gcc/cp/ChangeLog:

	PR c++/95158
	* class.c (lookup_vfn_in_binfo): New.
	* call.c (build_over_call): Use it.
	* cp-tree.h (resolves_to_fixed_type_p): Add default argument.
	(lookup_vfn_in_binfo): Declare.

gcc/testsuite/ChangeLog:

	PR c++/95158
	* g++.dg/template/virtual5.C: New test.