c: Allow variably-modified types in generic associations for C2Y

c: Allow variably-modified types in generic associations for C2Y

This implements part of N3348 to allow variably-modified types in
generic associations in C2Y and making it a pedantic warning before.
Allowing star * is not yet implemented.

gcc/c/ChangeLog:
	* c-parser.cc (c_parser_generic_selection): Change
	error_at to pedwarn_c23.

gcc/testsuite/ChangeLog:
	* gcc.dg/c11-generic-2.c: Adapt error message.
	* gcc.dg/c2y-generic-3.c: Adapt test.
	* gcc.dg/c2y-generic-4.c: New test.