Add support for device-modifiers for 'omp target device'.

'device_num' and 'ancestor' are now parsed on target device constructs for C,
C++, and Fortran (see OpenMP specification 5.0, p. 170). When 'ancestor' is
 used, then 'sorry, not supported' is output. Moreover, the restrictions for
'ancestor' are implemented (see OpenMP specification 5.0, p. 174f).

gcc/c/ChangeLog:

	* c-parser.c (c_parser_omp_clause_device): Parse device-modifiers 'device_num'
	and 'ancestor' in 'target device' clauses.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_omp_clause_device): Parse device-modifiers 'device_num'
	and 'ancestor' in 'target device' clauses.
	* semantics.c (finish_omp_clauses): Error handling. Constant device ids must
	evaluate to '1' if 'ancestor' is used.

gcc/fortran/ChangeLog:

	* gfortran.h: Add variable for 'ancestor' in struct gfc_omp_clauses.
	* openmp.c (gfc_match_omp_clauses): Parse device-modifiers 'device_num'
	and 'ancestor' in 'target device' clauses.
	* trans-openmp.c (gfc_trans_omp_clauses): Set OMP_CLAUSE_DEVICE_ANCESTOR.

gcc/ChangeLog:

	* gimplify.c (gimplify_scan_omp_clauses): Error handling. 'ancestor' only
	allowed on target constructs and only with particular other clauses.
	* omp-expand.c (expand_omp_target): Output of 'sorry, not supported' if
	'ancestor' is used.
	* omp-low.c (check_omp_nesting_restrictions): Error handling. No nested OpenMP
	structs when 'ancestor' is used.
	(scan_omp_1_stmt): No usage of OpenMP runtime routines in a target region when
	'ancestor' is used.
	* tree-pretty-print.c (dump_omp_clause): Append 'ancestor'.
	* tree.h (OMP_CLAUSE_DEVICE_ANCESTOR): Define macro.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/target-device-1.c: New test.
	* c-c++-common/gomp/target-device-2.c: New test.
	* c-c++-common/gomp/target-device-ancestor-1.c: New test.
	* c-c++-common/gomp/target-device-ancestor-2.c: New test.
	* c-c++-common/gomp/target-device-ancestor-3.c: New test.
	* c-c++-common/gomp/target-device-ancestor-4.c: New test.
	* gfortran.dg/gomp/target-device-1.f90: New test.
	* gfortran.dg/gomp/target-device-2.f90: New test.
	* gfortran.dg/gomp/target-device-ancestor-1.f90: New test.
	* gfortran.dg/gomp/target-device-ancestor-2.f90: New test.
	* gfortran.dg/gomp/target-device-ancestor-3.f90: New test.
	* gfortran.dg/gomp/target-device-ancestor-4.f90: New test.