Allow optional arguments to be used in the use_device OpenACC clause

Allow optional arguments to be used in the use_device OpenACC clause

Optional arguments should be treated as references rather than pointers
in the lowering.  However, for non-present arguments, this would result
in a null dereference, so conditionals need to be added to detect and
handle this.

	gcc/
	* omp-low.c (lower_omp_target): For use_device clauses, generate
	conditional statements to treat Fortran optional arguments like
	references if non-null, or propogate null arguments into offloaded
	code otherwise.

Reviewed-by: Julian Brown <julian@codesourcery.com>
(cherry picked from openacc-gcc-9-branch commit
4fba24cfbbd3738216a3551483744f82888741f3)