c++: C++20 DR 2237, disallow simple-template-id in cdtor.

This patch implements DR 2237 which says that a simple-template-id is
no longer valid as the declarator-id of a constructor or destructor;
see [diff.cpp17.class]#2.  It is not explicitly stated but out-of-line
destructors with a simple-template-id are also meant to be ill-formed
now.  (Out-of-line constructors like that are invalid since DR1435 I
think.)  This change only applies to C++20; it is not a DR against C++17.

I'm not crazy about the diagnostic in constructors but ISTM that
cp_parser_constructor_declarator_p shouldn't print errors.

	DR 2237
	* parser.c (cp_parser_unqualified_id): Reject simple-template-id as
	the declarator-id of a destructor.
	(cp_parser_constructor_declarator_p): Reject simple-template-id as
	the declarator-id of a constructor.

	* g++.dg/DRs/dr2237.C: New test.
	* g++.dg/parse/constructor2.C: Add dg-error for C++20.
	* g++.dg/parse/dtor12.C: Likewise.
	* g++.dg/parse/dtor4.C: Likewise.
	* g++.dg/template/dtor4.C: Adjust dg-error.
	* g++.dg/template/error34.C: Likewise.
	* g++.old-deja/g++.other/inline15.C: Only run for C++17 and lesses.
	* g++.old-deja/g++.pt/ctor2.C: Add dg-error for C++20.