re PR c++/13118 ([ABI] Missed covariant return thunk)

re PR c++/13118 ([ABI] Missed covariant return thunk)

cp:
	PR c++/13118
	* cp-tree.h (lang_decl_u): Add thunk_alias member.
	(THUNK_VIRTUAL_OFFSET): Must be a FUNCTION_DECL.
	(THUNK_ALIAS_P): Remove.
	(THUNK_ALIAS): Adjust.
	* class.c (update_vtable_entry_for_fn): Get the vbase within the
	overriding function's return type.
	(dump_thunk): Adjust THUNK_ALIAS printing.
	(build_vtbl_initializer): Adjust THUNK_ALIAS use.
	* method.c (make_thunk): Revert 12881 test change. Clear
	THUNK_ALIAS.
	(finish_thunk): Adjust THUNK_ALIAS setting.
	(use_thunk): Adjust THUNK_ALIAS use.
	* semantics.c (emit_associated_thunks): Likewise.
testsuite:
	PR c++/13118
	* g++.dg/abi/covariant3.C: New.

From-SVN: r74576