More explicit checking of which OMP constructs we're expecting

More explicit checking of which OMP constructs we're expecting

In particular, more precisely highlight what applies generally vs. the special
handling for the current 'parloops'-based OpenACC 'kernels' implementation.

	gcc/
	* omp-low.c (scan_sharing_clauses, scan_omp_for)
	(lower_oacc_reductions, lower_omp_target): More explicit checking
	of which OMP constructs we're expecting.