c++: CTAD within template argument [PR102933]

Here when checking for erroneous occurrences of 'auto' inside a template
argument (which is allowed by the concepts TS for class templates),
extract_autos_r picks up the CTAD placeholder for X{T{0}} which causes
check_auto_in_tmpl_args to reject this valid template argument.  This
patch fixes this by making extract_autos_r ignore CTAD placeholders.

However, it seems we don't need to call check_auto_in_tmpl_args at all
outside of the concepts TS since using 'auto' as a type-id is otherwise
rejected more generally at parse time.  So this patch makes the function
just exit early if !flag_concepts_ts.

Similarly, I think the concepts code paths in do_auto_deduction and
type_uses_auto are only necessary for the concepts TS, so this patch
also restricts these code paths accordingly.

	PR c++/102933

gcc/cp/ChangeLog:

	* parser.c (cp_parser_simple_type_specifier): Adjust diagnostic
	for using auto in parameter declaration.
	* pt.c (extract_autos_r): Ignore CTAD placeholders.
	(extract_autos): Use range-based for.
	(do_auto_deduction): Use extract_autos only for the concepts TS
	and not also for standard concepts.
	(type_uses_auto): Likewise with for_each_template_parm.
	(check_auto_in_tmpl_args): Just return false outside of the
	concepts TS.  Simplify.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class50.C: New test.
	* g++.dg/cpp2a/nontype-class50a.C: New test.

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