[flang][openacc] Add clause validity tests for the update directive

[clementval]

Summary

Add couple of clause validity tests for the update directive and check for
the restriction where at least self, host or device clause must appear on the directive.

[sameeranjoshi]

Looks Good Thanks.

[clementval]

Rebase

[clementval]

Revert to previous version after a bad rebase/update

flang/lib/Semantics/check-acc-structure.cpp

void AccStructureChecker::Leave(const parser::OpenACCStandaloneConstruct &x) {
   case llvm::acc::Directive::ACCD_exit_data:
   case llvm::acc::Directive::ACCD_set:
     // Restriction - 1117-1118 (ENTER DATA)
     // Restriction - 1161-1162 (EXIT DATA)
     // Restriction - 2254 (SET)
     CheckRequireAtLeastOneOf();
     break;
   case llvm::acc::Directive::ACCD_update:
+    // Restriction - line 2636
+    CheckRequireAtLeastOneOf();
     // Restriction - 2301
     CheckOnlyAllowedAfter(llvm::acc::Clause::ACCC_device_type,
         updateOnlyAllowedAfterDeviceTypeClauses);
     break;
   default:
     break;
   }
   dirContext_.pop_back();

flang/test/Semantics/acc-clause-validity.f90

program openacc_clause_validity
 
   !$acc data copyin(i)
   !$acc end data
 
   !$acc data copyin(i)
   !ERROR: Unmatched PARALLEL directive
   !$acc end parallel
 
+  !ERROR: At least one of DEVICE, HOST, SELF clause must appear on the UPDATE directive
+  !$acc update
+
   !$acc update self(a, f) host(g) device(h)
 
-  !$acc update device(i) device_type(*) async
+  !$acc update host(aa) async(1)
+
+  !$acc update device(bb) async(async1)
+
+  !ERROR: At most one ASYNC clause can appear on the UPDATE directive
+  !$acc update host(aa, bb) async(1) async(2)
+
+  !$acc update self(bb, cc(:)) wait(1)
+
+  !$acc update device(aa, bb, cc) wait(wait1)
+
+  !$acc update host(aa) host(bb) device(cc) wait(1,2)
+
+  !$acc update device(aa, cc) wait(wait1, wait2)
+
+  !$acc update device(aa) device_type(*) async
+
+  !$acc update host(bb) device_type(*) wait
+
+  !$acc update self(cc) device_type(1,2) async device_type(3) wait
+
+  !ERROR: At most one IF clause can appear on the UPDATE directive
+  !$acc update device(aa) if(.true.) if(ifCondition)
+
+  !ERROR: At most one IF_PRESENT clause can appear on the UPDATE directive
+  !$acc update device(bb) if_present if_present
 
   !ERROR: Clause IF is not allowed after clause DEVICE_TYPE on the UPDATE directive
   !$acc update device(i) device_type(*) if(.TRUE.)
 
   !$acc parallel
   !ERROR: INDEPENDENT and SEQ clauses are mutually exclusive and may not appear on the same LOOP directive
   !$acc loop seq independent
   do i = 1, N