OpenMP: Add -Wopenmp and use it

The new warning has two purposes: First, it makes clearer to the
user that it is about OpenMP and, secondly and more importantly,
it permits to use -Wno-openmp.

The newly added -Wopenmp is enabled by default and replaces the
'0' (always warning) in several OpenMP-related warning calls.
For code shared with OpenACC, it only uses OPT_Wopenmp for
'flag_openmp | flag_openmp_simd'.

gcc/c-family/ChangeLog:

	* c.opt (Wopenmp): Add, enable by default.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_omp_clause_num_threads,
	c_parser_omp_clause_num_tasks, c_parser_omp_clause_grainsize,
	c_parser_omp_clause_priority, c_parser_omp_clause_schedule,
	c_parser_omp_clause_num_teams, c_parser_omp_clause_thread_limit,
	c_parser_omp_clause_dist_schedule, c_parser_omp_depobj,
	c_parser_omp_scan_loop_body, c_parser_omp_assumption_clauses):
	Add OPT_Wopenmp to warning_at.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_omp_clause_dist_schedule,
	cp_parser_omp_scan_loop_body, cp_parser_omp_assumption_clauses,
	cp_parser_omp_depobj): Add OPT_Wopenmp to warning_at.
	* semantics.cc (finish_omp_clauses): Likewise.

gcc/ChangeLog:

	* doc/invoke.texi (-Wopenmp): Add.
	* gimplify.cc (gimplify_omp_for): Add OPT_Wopenmp to warning_at.
	* omp-expand.cc (expand_omp_ordered_sink): Likewise.
	* omp-general.cc (omp_check_context_selector): Likewise.
	* omp-low.cc (scan_omp_for, check_omp_nesting_restrictions,
	lower_omp_ordered_clauses): Likewise.
	* omp-simd-clone.cc (simd_clone_clauses_extract): Likewise.

gcc/fortran/ChangeLog:

	* lang.opt (Wopenmp): Add, enabled by dafault and documented in C.
	* openmp.cc (gfc_match_omp_declare_target, resolve_positive_int_expr,
	resolve_nonnegative_int_expr, resolve_omp_clauses,
	gfc_resolve_omp_do_blocks): Use OPT_Wopenmp with gfc_warning{,_now}.