[Flang] [OpenMP] Refine parser restrictions for OMP TARGET UPDATE clauses.

[raghavendhra]

Summary

In Parser, move some clauses of OMP TARGET UPDATE to allowedOnceClauses so that restrictions will be imposed.

[raghavendhra]

Rebasing the patch.

[kiranchandramohan]

LG.

flang/test/Semantics/OpenMP/omp-device-constructs.f90

program main
   !$omp target exit data map(delete:a)
 
   !ERROR: At most one DEVICE clause can appear on the TARGET EXIT DATA directive
   !$omp target exit data map(from:a) device(0) device(1)
 
   !ERROR: Only the FROM, RELEASE, DELETE map types are permitted for MAP clauses on the TARGET EXIT DATA directive
   !$omp target exit data map(to:a)
 
+  !$omp target update if(.true.) device(1) to(a) from(b) depend(inout:c) nowait
+
+  !ERROR: At most one IF clause can appear on the TARGET UPDATE directive
+  !$omp target update to(a) if(.true.) if(.false.)
+
+  !ERROR: At most one DEVICE clause can appear on the TARGET UPDATE directive
+  !$omp target update device(0) device(1) from(b)
+
   !$omp target
   !ERROR: `DISTRIBUTE` region has to be strictly nested inside `TEAMS` region.
   !$omp distribute
   do i = 1, N
      a = 3.14
   enddo
   !$omp end distribute
   !$omp end target

llvm/include/llvm/Frontend/OpenMP/OMP.td

let allowedOnceClauses = [
     VersionedClause<OMPC_Schedule>,
     VersionedClause<OMPC_Collapse>,
     VersionedClause<OMPC_Ordered>,
     VersionedClause<OMPC_NoWait>
   ];
 }
 def OMP_TargetUpdate : Directive<"target update"> {
   let allowedClauses = [
-    VersionedClause<OMPC_If>,
-    VersionedClause<OMPC_Device>,
     VersionedClause<OMPC_To>,
     VersionedClause<OMPC_From>,
-    VersionedClause<OMPC_NoWait>,
     VersionedClause<OMPC_Depend>
   ];
+  let allowedOnceClauses = [
+    VersionedClause<OMPC_Device>,
+    VersionedClause<OMPC_If>,
+    VersionedClause<OMPC_NoWait>
+  ];
 }
 def OMP_ParallelFor : Directive<"parallel for"> {
   let allowedClauses = [
     VersionedClause<OMPC_If>,
     VersionedClause<OMPC_NumThreads>,
     VersionedClause<OMPC_Default>,
     VersionedClause<OMPC_ProcBind>,
     VersionedClause<OMPC_Private>,