c++: Implement CWG 625: Use of auto as template-arg [PR97479]

c++: Implement CWG 625: Use of auto as template-arg [PR97479]

This patch implements CWG 625 which prohibits using auto in a template
argument.  A few tests used this construction.  Since this usage was
allowed by the Concepts TS, we only give an error in C++20.

gcc/cp/ChangeLog:

	DR 625
	PR c++/97479
	* parser.c (cp_parser_type_id_1): Reject using auto as
	a template-argument in C++20.

gcc/testsuite/ChangeLog:

	DR 625
	PR c++/97479
	* g++.dg/cpp0x/auto3.C: Update dg-error.
	* g++.dg/cpp0x/auto9.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-2.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-3.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979.C: Likewise.
	* g++.dg/DRs/dr625.C: New test.