OpenMP: C front-end support for dispatch + adjust_args

This patch adds support to the C front-end to parse the `dispatch` construct and
the `adjust_args` clause. It also includes some common C/C++ bits for pragmas
and attributes.

Additional common C/C++ testcases are in a later patch in the series.

gcc/c-family/ChangeLog:

	* c-attribs.cc (c_common_gnu_attributes): Add attribute for adjust_args
	need_device_ptr.
	* c-omp.cc (c_omp_directives): Uncomment dispatch.
	* c-pragma.cc (omp_pragmas): Add dispatch.
	* c-pragma.h (enum pragma_kind): Add PRAGMA_OMP_DISPATCH.
	(enum pragma_omp_clause): Add PRAGMA_OMP_CLAUSE_NOCONTEXT and
	PRAGMA_OMP_CLAUSE_NOVARIANTS.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_omp_dispatch): New function.
	(c_parser_omp_clause_name): Handle nocontext and novariants clauses.
	(c_parser_omp_clause_novariants): New function.
	(c_parser_omp_clause_nocontext): Likewise.
	(c_parser_omp_all_clauses): Handle nocontext and novariants clauses.
	(c_parser_omp_dispatch_body): New function adapted from
	c_parser_expr_no_commas.
	(OMP_DISPATCH_CLAUSE_MASK): Define.
	(c_parser_omp_dispatch): New function.
	(c_finish_omp_declare_variant): Parse adjust_args.
	(c_parser_omp_construct): Handle PRAGMA_OMP_DISPATCH.
	* c-typeck.cc (c_finish_omp_clauses): Handle OMP_CLAUSE_NOVARIANTS and
	OMP_CLAUSE_NOCONTEXT.

gcc/testsuite/ChangeLog:

	* gcc.dg/gomp/adjust-args-1.c: New test.
	* gcc.dg/gomp/dispatch-1.c: New test.
	* gcc.dg/gomp/dispatch-2.c: New test.
	* gcc.dg/gomp/dispatch-3.c: New test.
	* gcc.dg/gomp/dispatch-4.c: New test.
	* gcc.dg/gomp/dispatch-5.c: New test.

Author: parras

gcc/c-family/c-attribs.cc

@@ -572,6 +572,9 @@ const struct attribute_spec c_common_gnu_attributes[] =
 			      handle_omp_declare_variant_attribute, NULL },
   { "omp declare variant variant", 0, -1, true,  false, false, false,
 			      handle_omp_declare_variant_attribute, NULL },
+  { "omp declare variant adjust_args need_device_ptr", 0, -1, true,  false,
+			      false, false,
+			      handle_omp_declare_variant_attribute, NULL },
   { "simd",		      0, 1, true,  false, false, false,
 			      handle_simd_attribute, NULL },
   { "omp declare target",     0, -1, true, false, false, false,

gcc/c-family/c-omp.cc

@@ -4300,8 +4300,8 @@ const struct c_omp_directive c_omp_directives[] = {
     C_OMP_DIR_DECLARATIVE, false },
   { "depobj", nullptr, nullptr, PRAGMA_OMP_DEPOBJ,
     C_OMP_DIR_STANDALONE, false },
-  /* { "dispatch", nullptr, nullptr, PRAGMA_OMP_DISPATCH,
-    C_OMP_DIR_CONSTRUCT, false },  */
+  { "dispatch", nullptr, nullptr, PRAGMA_OMP_DISPATCH,
+    C_OMP_DIR_DECLARATIVE, false },
   { "distribute", nullptr, nullptr, PRAGMA_OMP_DISTRIBUTE,
     C_OMP_DIR_CONSTRUCT, true },
   { "end", "assumes", nullptr, PRAGMA_OMP_END,

gcc/c-family/c-pragma.cc

@@ -1525,6 +1525,7 @@ static const struct omp_pragma_def omp_pragmas[] = {
   { "cancellation", PRAGMA_OMP_CANCELLATION_POINT },
   { "critical", PRAGMA_OMP_CRITICAL },
   { "depobj", PRAGMA_OMP_DEPOBJ },
+  { "dispatch", PRAGMA_OMP_DISPATCH },
   { "error", PRAGMA_OMP_ERROR },
   { "end", PRAGMA_OMP_END },
   { "flush", PRAGMA_OMP_FLUSH },

gcc/c-family/c-pragma.h

@@ -55,6 +55,7 @@ enum pragma_kind {
   PRAGMA_OMP_CRITICAL,
   PRAGMA_OMP_DECLARE,
   PRAGMA_OMP_DEPOBJ,
+  PRAGMA_OMP_DISPATCH,
   PRAGMA_OMP_DISTRIBUTE,
   PRAGMA_OMP_ERROR,
   PRAGMA_OMP_END,
@@ -135,9 +136,11 @@ enum pragma_omp_clause {
   PRAGMA_OMP_CLAUSE_LINK,
   PRAGMA_OMP_CLAUSE_MAP,
   PRAGMA_OMP_CLAUSE_MERGEABLE,
+  PRAGMA_OMP_CLAUSE_NOCONTEXT,
   PRAGMA_OMP_CLAUSE_NOGROUP,
   PRAGMA_OMP_CLAUSE_NONTEMPORAL,
   PRAGMA_OMP_CLAUSE_NOTINBRANCH,
+  PRAGMA_OMP_CLAUSE_NOVARIANTS,
   PRAGMA_OMP_CLAUSE_NOWAIT,
   PRAGMA_OMP_CLAUSE_NUM_TASKS,
   PRAGMA_OMP_CLAUSE_NUM_TEAMS,