[flang][openacc] Fix clause restriction for exit data directive

[clementval]

Summary

Restriction on clauses for the EXIT DATA directive were not fully correct.
This patch fixes the situation. The async, if and finalize clauses are allowed
only once.

[SouraVX]

LGTM! Thanks!

flang/test/Semantics/OpenACC/acc-data.f90

program openacc_data_validity
 
   !ERROR: Modifier is not allowed for the COPYOUT clause on the EXIT DATA directive
   !$acc exit data copyout(zero: i)
 
   !$acc exit data delete(aa)
 
   !$acc exit data delete(aa) finalize
 
+  !ERROR: At most one FINALIZE clause can appear on the EXIT DATA directive
+  !$acc exit data delete(aa) finalize finalize
+
   !$acc exit data detach(cc)
 
   !$acc exit data copyout(bb)
 
   !$acc exit data delete(aa) if(.TRUE.)
 
   !$acc exit data delete(aa) if(ifCondition)
 
+  !ERROR: At most one IF clause can appear on the EXIT DATA directive
+  !$acc exit data delete(aa) if(ifCondition) if(.TRUE.)
+
   !$acc exit data delete(aa) async
 
+  !ERROR: At most one ASYNC clause can appear on the EXIT DATA directive
+  !$acc exit data delete(aa) async async
+
   !$acc exit data delete(aa) async(async1)
 
   !$acc exit data delete(aa) async(1)
 
   !$acc exit data delete(aa) wait(1)
 
   !$acc exit data delete(aa) wait(wait1)
 

llvm/include/llvm/Frontend/OpenACC/ACC.td

let requiredClauses = [
     VersionedClause<ACCC_Create>,
     VersionedClause<ACCC_Copyin>
   ];
 }
 
 // 2.14.7
 def ACC_ExitData : Directive<"exit data"> {
   let allowedClauses = [
+    VersionedClause<ACCC_Wait>
+  ];
+  let allowedOnceClauses = [
     VersionedClause<ACCC_Async>,
     VersionedClause<ACCC_If>,
-    VersionedClause<ACCC_Wait>,
     VersionedClause<ACCC_Finalize>
   ];
   let requiredClauses = [
     VersionedClause<ACCC_Copyout>,
     VersionedClause<ACCC_Delete>,
     VersionedClause<ACCC_Detach>
   ];
 }