PR c++/79316 - default argument in deduction guide

PR c++/79316 - default argument in deduction guide

	PR c++/79350 - explicit deduction guide
	* parser.c (cp_parser_constructor_declarator_p)
	(cp_parser_direct_declarator): Parse deduction guides more like
	constructors.
	* cp-tree.h (enum special_function_kind): Add sfk_deduction_guide.
	* tree.c (special_function_p): Return it.
	* decl.c (check_special_function_return_type): Handle it.
	(grokdeclarator, grokfndecl): Adjust.
	(cp_finish_decl): Pass flags to do_auto_deduction.
	* error.c (dump_decl_name): Use TFF_UNQUALIFIED_NAME.
	* pt.c (dguide_name_p): Take a const_tree.
	(do_class_deduction): Handle explicit.
	(do_auto_deduction): Pass flags through.
	(build_deduction_guide): Copy explicit flag.

From-SVN: r245314