c++: Parameter pack in requires parameter list [PR94808]

c++: Parameter pack in requires parameter list [PR94808]

When printing the substituted parameter list of a requires-expression as
part of the "in requirements with ..." context line during concepts
diagnostics, we weren't considering that substitution into a parameter
pack can yield zero or multiple parameters.

This patch changes the way we print the parameter list of a
requires-expression in print_requires_expression_info.  We now print the
dependent form of the parameter list (along with its template parameter
mapping) instead of printing its substituted form.  Besides being an
improvement in its own, this also sidesteps the substitution issue in the
PR altogether.

gcc/cp/ChangeLog:

	PR c++/94808
	* error.c (print_requires_expression_info): Print the dependent
	form of the parameter list with its template parameter mapping,
	rather than printing the substituted form.

gcc/testsuite/ChangeLog:

	PR c++/94808
	* g++.dg/concepts/diagnostic12.C: New test.
	* g++.dg/concepts/diagnostic5.C: Adjust dg-message.