c++: bogus error with union in qualified name [PR83469]

While working on Reflection I noticed that we reject:

  union U { int i; };
  constexpr auto r = ^^typename ::U;

which is due to PR83469.  Andrew P. posted a patch in 2021:
https://gcc.gnu.org/pipermail/gcc-patches/2021-December/586344.html
for which I had some comments but an updated patch never came.

~~
There are a few issues here with typenames and unions (and even struct
keywords with unions). First in cp_parser_check_class_key,
we need to allow typenames to name union types and union key
to be able to use with typenames.

The next issue is we need to record if we had a union key,
right now we just record it was a struct/class/typename one
which is wrong.
~~

This patch is an updated and cleaned up version; I've also addressed
a missing bit in pt.cc.

	PR c++/83469
	PR c++/93809

gcc/cp/ChangeLog:

	* cp-tree.h (UNION_TYPE_P): Define.
	(TYPENAME_IS_UNION_P): Define.
	* decl.cc (struct typename_info): Add union_p field.
	(struct typename_hasher::equal): Compare union_p field.
	(build_typename_type): Use ti.union_p for union_type.  Set
	TYPENAME_IS_UNION_P.
	* error.cc (dump_type) <case TYPENAME_TYPE>: Handle
	TYPENAME_IS_UNION_P.
	* module.cc (trees_out::type_node): Likewise.
	* parser.cc (cp_parser_check_class_key): Allow typename key for union
	types and allow union keyword for typename types.
	* pt.cc (tsubst) <case TYPENAME_TYPE>: Don't conflate unions with
	class_type.  For TYPENAME_IS_CLASS_P, check NON_UNION_CLASS_TYPE_P
	rather than CLASS_TYPE_P.  Add TYPENAME_IS_UNION_P handling.

gcc/testsuite/ChangeLog:

	* g++.dg/template/error45.C: Adjust dg-error.
	* g++.dg/warn/Wredundant-tags-3.C: Remove xfail.
	* g++.dg/parse/union1.C: New test.
	* g++.dg/parse/union2.C: New test.
	* g++.dg/parse/union3.C: New test.
	* g++.dg/parse/union4.C: New test.
	* g++.dg/parse/union5.C: New test.
	* g++.dg/parse/union6.C: New test.

Co-authored-by: Andrew Pinski <quic_apinski@quicinc.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

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