c++: look for empty base at specific offset [PR109678]

c++: look for empty base at specific offset [PR109678]

While looking at the empty base handling for 109678, it occurred to me that
we ought to be able to look for an empty base at a specific offset, not just
in general.

	PR c++/109678

gcc/cp/ChangeLog:

	* cp-tree.h (lookup_base): Add offset parm.
	* constexpr.cc (cxx_fold_indirect_ref_1): Pass it.
	* search.cc (struct lookup_base_data_s): Add offset.
	(dfs_lookup_base): Handle it.
	(lookup_base): Pass it.