c++: Diagnose when "requires" is used instead of "requires requires" [PR94306]

c++: Diagnose when "requires" is used instead of "requires requires" [PR94306]

This adds support to detect and recover from the case where an opening brace
immediately follows the start of a requires-clause.  So rather than emitting the
error

  error: expected primary-expression before '{' token

followed by a slew of irrevelant errors, we now assume the user had intended to
write "requires requires {" and diagnose and recover accordingly.

gcc/cp/ChangeLog:

	PR c++/94306
	* parser.c (cp_parser_requires_clause_opt): Diagnose and recover from
	"requires {" when "requires requires {" was probably intended.

gcc/testsuite/ChangeLog:

	PR c++/94306
	* g++.dg/concepts/diagnostic8.C: New test.