c++: Fix constexpr access to union member through pointer-to-member [PR98122]

c++: Fix constexpr access to union member through pointer-to-member [PR98122]

We currently incorrectly reject the first testcase, because
cxx_fold_indirect_ref_1 doesn't attempt to handle UNION_TYPEs.
As the second testcase shows, it isn't that easy, because I believe we need
to take into account the active member and prefer that active member over
other members, because if we pick a non-active one, we might reject valid
programs.

2020-12-05  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98122
	* constexpr.c (cxx_union_active_member): New function.
	(cxx_fold_indirect_ref_1): Add ctx argument, pass it through to
	recursive call.  Handle UNION_TYPE.
	(cxx_fold_indirect_ref): Add ctx argument, pass it to recursive calls
	and cxx_fold_indirect_ref_1.
	(cxx_eval_indirect_ref): Adjust cxx_fold_indirect_ref calls.

	* g++.dg/cpp1y/constexpr-98122.C: New test.
	* g++.dg/cpp2a/constexpr-98122.C: New test.