Add a "combined" flag for "acc kernels loop" etc directives.

Add a "combined" flag for "acc kernels loop" etc directives.

2020-08-19  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/
	* tree.h (OACC_LOOP_COMBINED): New.

	gcc/c/
	* c-parser.cc (c_parser_oacc_loop): Set OACC_LOOP_COMBINED.

	gcc/cp/
	* parser.cc (cp_parser_oacc_loop): Set OACC_LOOP_COMBINED.

	gcc/fortran/
	* trans-openmp.cc (gfc_trans_omp_do): Add combined parameter,
	use it to set OACC_LOOP_COMBINED.  Update all call sites.