c++: Fix initializing empty base from prvalue [PR97597]

c++: Fix initializing empty base from prvalue [PR97597]

unsafe_return_slot_p wasn't recognizing an empty base as
potentially-overlapping, which it definitely is.

The change to build_base_path is to make the virtual conversion also
recognized by is_empty_base_ref; unsafe_return_slot_p doesn't to handle
virtual conversions, but hypothetical future callers might.

gcc/cp/ChangeLog:

	PR c++/97597
	* class.c (is_empty_base_ref): New.
	(build_base_path): Add NOP_EXPR after offset.
	* cp-tree.h (is_empty_base_ref): Declare it.
	* call.c (unsafe_return_slot_p): Call it.

gcc/testsuite/ChangeLog:

	PR c++/97597
	* g++.dg/init/empty3.C: New test.