OpenMP/C++: Use STRIP_REFERENCE_REF to fix declare variant with reference-returning...

OpenMP/C++: Use STRIP_REFERENCE_REF to fix declare variant with reference-returning functions

As Jakub suggested, use STRIP_REFERENCE_REF instead of doing it manually
as r15-4800-geb828a1e380e7b did.

gcc/cp/ChangeLog:

	* decl.cc (omp_declare_variant_finalize_one): Use STRIP_REFERENCE_REF
	instead of doing it manually.