OpenMP: Pass a 3-way flag to omp_check_context_selector instead of a bool.

OpenMP: Pass a 3-way flag to omp_check_context_selector instead of a bool.

The OpenMP "begin declare variant" directive has slightly different
requirements for context selectors than regular "declare variant", so
something more than a bool is required to tell the error-checking routine
what to check.

gcc/ChangeLog
	* omp-general.cc (omp_check_context_selector): Change
	metadirective_p argument to a 3-way flag.  Add extra check for
	OMP_CTX_BEGIN_DECLARE_VARIANT.
	* omp-general.h (enum omp_ctx_directive): New.
	(omp_check_context_selector): Adjust declaration.

gcc/c/ChangeLog
	* c-parser.cc (c_finish_omp_declare_variant): Update call to
	omp_check_context_selector.
	(c_parser_omp_metadirective): Likewise.

gcc/cp/ChangeLog
	* parser.cc (cp_finish_omp_declare_variant): Update call to
	omp_check_context_selector.
	(cp_parser_omp_metadirective): Likewise.

gcc/fortran/ChangeLog
	* trans-openmp.cc (gfc_trans_omp_declare_variant): Update call to
	omp_check_context_selector.
	(gfc_trans_omp_metadirective): Likewise.