conversion_path points to the base where we found the using-declaration, not
where the function is actually a member; look up the actual base.  And then
maybe look back to the derived class if the base is primary.

gcc/cp/ChangeLog:

	PR c++/95719
	* call.c (build_over_call): Look up the overrider in base_binfo.
	* class.c (lookup_vfn_in_binfo): Look through BINFO_PRIMARY_P.

gcc/testsuite/ChangeLog:

	PR c++/95719
	* g++.dg/tree-ssa/final4.C: New test.

(cherry picked from commit 7d6baf68fe22b6ef5b1d6fabbef97c0e1b4d7abf)