c++: bogus error w/ tentative type parse of concept-id [PR98394]

c++: bogus error w/ tentative type parse of concept-id [PR98394]

Here when tentatively parsing the if condition as a declaration, we try
to treat C<1> as the start of a constrained placeholder type, which we
quickly reject because C doesn't accept a type as its first argument.
But since we're parsing tentatively, we shouldn't emit an error in this
case.

In passing, also fix PR85846 by only overriding 'tentative' to false when
given a concept-name, and not also when given a concept-id that has an empty
argument list.

	PR c++/98394
	PR c++/85846

gcc/cp/ChangeLog:

	* parser.c (cp_parser_placeholder_type_specifier): Declare
	static.  Don't override tentative to false when tmpl is a
	concept-id with empty argument list.  Don't emit a "does not
	constrain a type" error when tentative.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-pr98394.C: New test.
	* g++.dg/cpp2a/concepts-pr85846.C: New test.