Implement multiple 'auto' feature from Concepts TS.

Implement multiple 'auto' feature from Concepts TS.

	* parser.c (cp_parser_type_id_1): Allow 'auto' if -fconcepts.
	(cp_parser_template_type_arg): Likewise.
	(get_concept_from_constraint): Split out most logic to...
	* constraint.cc (placeholder_extract_concept_and_args): ...here.
	(equivalent_placeholder_constraints, hash_placeholder_constraint): New.
	* cxx-pretty-print.c (pp_cxx_constrained_type_spec): New.
	* cxx-pretty-print.h: Declare it.
	* error.c (dump_type) [TEMPLATE_TYPE_PARM]: Call it.
	* pt.c (is_auto_r, extract_autos_r, extract_autos, auto_hash): New.
	(type_uses_auto): Use is_auto_r.
	(do_auto_deduction): Handle multiple 'auto's if -fconcepts.
	* typeck.c (structural_comptypes) [TEMPLATE_TYPE_PARM]: Compare
	constraints.

From-SVN: r229629