c++: diagnose this specifier in requires expr [PR116798]

c++: diagnose this specifier in requires expr [PR116798]

We don't detect an explicit object parameter in a requires expression.
We can get there by way of requires-expression -> requirement-parameter-list
-> parameter-declaration-clause -> ... -> parameter-declaration with
this[opt].  But [dcl.fct]/5 doesn't allow an explicit object parameter
in this context.  So let's fix it like r14-9033 and not like r14-8832.

	PR c++/116798

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_parameter_declaration): Detect an explicit
	object parameter in a requires expression.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/explicit-obj-diagnostics12.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>