Further changes for the OpenACC 'if_present' clause on the 'host_data' construct

	gcc/
	* tree.h (OMP_CLAUSE_USE_DEVICE_PTR_IF_PRESENT): New definition.
	* tree-core.h: Document it.
	* gimplify.c (gimplify_omp_workshare): Set it.
	* omp-low.c (lower_omp_target): Use it.
	* tree-pretty-print.c (dump_omp_clause): Print it.
	gcc/testsuite/
	* c-c++-common/goacc/host_data-1.c: Extend.
	* gfortran.dg/goacc/host_data-tree.f95: Likewise.

	gcc/
	* omp-low.c (lower_omp_target) <OMP_CLAUSE_USE_DEVICE_PTR etc.>:
	Assert that for OpenACC we always have 'GOMP_MAP_USE_DEVICE_PTR'.

	libgomp/
	* target.c (gomp_map_vars_internal)
	<GOMP_MAP_USE_DEVICE_PTR_IF_PRESENT>: Clean up/elaborate code
	paths.

From-SVN: r280149