OpenMP: Unify representation of name-list properties.

OpenMP: Unify representation of name-list properties.

Previously, name-list properties specified as identifiers were stored
in the TREE_PURPOSE/OMP_TP_NAME slot, while those specified as strings
were stored in the TREE_VALUE/OMP_TP_VALUE slot.  This patch puts both
representations in OMP_TP_VALUE with a magic cookie in OMP_TP_NAME.

gcc/ChangeLog
	* omp-general.h (OMP_TP_NAMELIST_NODE): New.
	* omp-general.cc (omp_context_name_list_prop): Move earlier
	in the file, and adjust for new representation.
	(omp_check_context_selector): Adjust this too.
	(omp_context_selector_props_compare): Likewise.

gcc/c/ChangeLog
	* c-parser.cc (c_parser_omp_context_selector): Adjust for new
	namelist property representation.

gcc/cp/ChangeLog
	* parser.cc (cp_parser_omp_context_selector): Adjust for new
	namelist property representation.
	* pt.cc (tsubst_attribute): Likewise.

gcc/fortran/ChangeLog
	* trans-openmp.cc (gfc_trans_omp_declare_varaint): Adjust for
	new namelist property representation.