OpenMP/PolyInt: Pass poly-int structures by address to OMP libs.

Currently poly-int type structures are passed by value to OpenMP runtime
functions for shared clauses etc.  This patch improves on this by passing
around poly-int structures by address to avoid copy-overhead.

gcc/ChangeLog:

	* omp-low.cc (use_pointer_for_field): Use pointer if the OMP data
	structure's field type is a poly-int.