c++: further -Wdangling-reference refinement [PR107532]

c++: further -Wdangling-reference refinement [PR107532]

Based on <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107532#c24>,
it seems like we should treat *any* class with a reference member
as a reference wrapper.  To suppress the warning in

  int i = 42;
  auto const& v = std::get<0>(std::tuple<int&>(i));

we have to look into base classes as well.  For std::tuple, this means
that we have to check the _Head_base subobject, which is a non-direct
base class of std::tuple.  So I've employed a DFS walk.

	PR c++/107532

gcc/cp/ChangeLog:

	* call.cc (class_has_reference_member_p): New.
	(class_has_reference_member_p_r): New.
	(reference_like_class_p): Don't look for a specific constructor.
	Use a DFS walk with class_has_reference_member_p_r.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference11.C: New test.
	* g++.dg/warn/Wdangling-reference12.C: New test.