c++: recognize class-scope non-template dguides [PR79501]

c++: recognize class-scope non-template dguides [PR79501]

It looks like we still don't recognize class-scope non-template
deduction guides even after r12-2260.   This is because deduction guides
are tagged as such in cp_parser_init_declarator after calling
cp_parser_declarator, but in cp_parser_member_declaration we call
cp_parser_declarator directly.

So let's tag them in cp_parser_member_declaration as well.

	PR c++/79501

gcc/cp/ChangeLog:

	* parser.c (maybe_adjust_declarator_for_dguide): New, split
	out from ...
	(cp_parser_init_declarator): ... here.
	(cp_parser_member_declaration): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction98.C: New test.