[flang][OpenMP] Mark atomic clauses as unique

The current implementation of the ATOMIC construct handles these clauses
individually, and this change does not have an observable effect. At the
same time these clauses are unique as per the OpenMP spec, and this patch
reflects that in the OMP.td file.

Author: kparzysz

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

@@ -602,22 +602,20 @@ def OMP_Assume : Directive<"assume"> {
   ];
 }
 def OMP_Atomic : Directive<"atomic"> {
-  let allowedClauses = [
-    VersionedClause<OMPC_Capture>,
-    VersionedClause<OMPC_Compare, 51>,
-    VersionedClause<OMPC_Read>,
-    VersionedClause<OMPC_Update>,
-    VersionedClause<OMPC_Write>,
-  ];
   let allowedOnceClauses = [
     VersionedClause<OMPC_AcqRel, 50>,
     VersionedClause<OMPC_Acquire, 50>,
+    VersionedClause<OMPC_Capture>,
+    VersionedClause<OMPC_Compare, 51>,
     VersionedClause<OMPC_Fail, 51>,
     VersionedClause<OMPC_Hint, 50>,
+    VersionedClause<OMPC_Read>,
     VersionedClause<OMPC_Relaxed, 50>,
     VersionedClause<OMPC_Release, 50>,
     VersionedClause<OMPC_SeqCst>,
+    VersionedClause<OMPC_Update>,
     VersionedClause<OMPC_Weak, 51>,
+    VersionedClause<OMPC_Write>,
   ];
   let association = AS_Block;
   let category = CA_Executable;
@@ -668,7 +666,7 @@ def OMP_CancellationPoint : Directive<"cancellation point"> {
   let category = CA_Executable;
 }
 def OMP_Critical : Directive<"critical"> {
-  let allowedClauses = [
+  let allowedOnceClauses = [
     VersionedClause<OMPC_Hint>,
   ];
   let association = AS_Block;