OpenACC 2.7: front-end support for readonly modifier

This patch implements the front-end support for the 'readonly' modifier for the
OpenACC 'copyin' clause and 'cache' directive.

This currently only includes front-end parsing for C/C++/Fortran and setting of
new bits OMP_CLAUSE_MAP_READONLY, OMP_CLAUSE__CACHE__READONLY. Further linking
of these bits to points-to analysis and/or utilization of read-only memory in
accelerator target are for later patches.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_oacc_data_clause): Add parsing support for
	'readonly' modifier, set OMP_CLAUSE_MAP_READONLY if readonly modifier
	found, update comments.
	(c_parser_oacc_cache): Add parsing support for 'readonly' modifier,
	set OMP_CLAUSE__CACHE__READONLY if readonly modifier found, update
	comments.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_oacc_data_clause): Add parsing support for
	'readonly' modifier, set OMP_CLAUSE_MAP_READONLY if readonly modifier
	found, update comments.
	(cp_parser_oacc_cache): Add parsing support for 'readonly' modifier,
	set OMP_CLAUSE__CACHE__READONLY if readonly modifier found, update
	comments.

gcc/fortran/ChangeLog:

	* dump-parse-tree.cc (show_omp_namelist): Print "readonly," for
	OMP_LIST_MAP and OMP_LIST_CACHE if n->u.map.readonly is set.
	Adjust 'n->u.map_op' to 'n->u.map.op'.
	* gfortran.h (typedef struct gfc_omp_namelist): Adjust map_op as
	'ENUM_BITFIELD (gfc_omp_map_op) op:8', add 'bool readonly' field,
	change to named struct field 'map'.
	* openmp.cc (gfc_match_omp_map_clause): Adjust 'n->u.map_op' to
	'n->u.map.op'.
	(gfc_match_omp_clause_reduction): Likewise.
	(gfc_match_omp_clauses): Add readonly modifier parsing for OpenACC
	copyin clause, set 'n->u.map.op' and 'n->u.map.readonly' for parsed
	clause. Adjust 'n->u.map_op' to 'n->u.map.op'.
	(gfc_match_oacc_declare): Adjust 'n->u.map_op' to 'n->u.map.op'.
	(gfc_match_oacc_cache): Add readonly modifier parsing for OpenACC
	cache directive.
	(resolve_omp_clauses): Adjust 'n->u.map_op' to 'n->u.map.op'.
	* trans-decl.cc (add_clause): Adjust 'n->u.map_op' to 'n->u.map.op'.
	(finish_oacc_declare): Likewise.
	* trans-openmp.cc (gfc_trans_omp_clauses): Set OMP_CLAUSE_MAP_READONLY,
	OMP_CLAUSE__CACHE__READONLY to 1 when readonly is set. Adjust
	'n->u.map_op' to 'n->u.map.op'.
	(gfc_add_clause_implicitly): Adjust 'n->u.map_op' to 'n->u.map.op'.

gcc/ChangeLog:

	* tree.h (OMP_CLAUSE_MAP_READONLY): New macro.
	(OMP_CLAUSE__CACHE__READONLY): New macro.
	* tree-core.h (struct GTY(()) tree_base): Adjust comments for new
	uses of readonly_flag bit in OMP_CLAUSE_MAP_READONLY and
	OMP_CLAUSE__CACHE__READONLY.
	* tree-pretty-print.cc (dump_omp_clause): Add support for printing
	OMP_CLAUSE_MAP_READONLY and OMP_CLAUSE__CACHE__READONLY.

gcc/testsuite/ChangeLog:

	* c-c++-common/goacc/readonly-1.c: New test.
	* gfortran.dg/goacc/readonly-1.f90: New test.