c++: omp declare variant tweak

c++: omp declare variant tweak

In r15-6707 I changed this function to use build_stub_object to more simply
produce the right type, but it occurs to me that forward_parm would be even
better, specifically for the diagnostic.

This changes nothing with respect to PR118791.

gcc/cp/ChangeLog:

	* decl.cc (omp_declare_variant_finalize_one): Use forward_parm.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/declare-variant-3.C: Adjust diagnostic.
	* g++.dg/gomp/declare-variant-5.C: Adjust diagnostic.