[OMP] Standardize on 'omp_privatize_by_reference'

... instead of 'omp_is_reference' vs.
'lang_hooks.decls.omp_privatize_by_reference'.

	gcc/
	* omp-general.h (omp_is_reference): Rename to...
	(omp_privatize_by_reference): ... this.  Adjust all users...
	* omp-general.c: ... here, ...
	* gimplify.c: ... here, ...
	* omp-expand.c: ... here, ...
	* omp-low.c: ... here.