openacc: GOMP_MAP_ATTACH handling in find_group_last

openacc: GOMP_MAP_ATTACH handling in find_group_last

Arrange for GOMP_MAP_ATTACH to be grouped together with a preceding
GOMP_MAP_TO_PSET or other "to" data movement clause, except in cases
where an explicit "attach" clause is used.

2020-07-09  Julian Brown  <julian@codesourcery.com>

include/
	* gomp-constants.h (gomp_map_kind): Update comment for GOMP_MAP_TO_PSET.

libgomp/
	* oacc-mem.c (find_group_last): Group data-movement clauses
	(GOMP_MAP_TO_PSET, GOMP_MAP_TO, etc.) together with a subsequent
	GOMP_MAP_ATTACH.  Allow standalone GOMP_MAP_ATTACH also.