[flang][openacc] Restrict branching out of data construct

[clementval]

Summary

This restriction is not clearly stated in the OpenACC standard
but all other compilers enforce it and there no explanation to
what should happen if there is a branching out of the construct.
Add the restriction for now.

[vzakhari]

LGTM. Thank you!

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

case llvm::acc::Directive::ACCD_serial:
     CheckOnlyAllowedAfter(llvm::acc::Clause::ACCC_device_type,
         computeConstructOnlyAllowedAfterDeviceTypeClauses);
     // Restriction - line 1001
     CheckNoBranching(block, GetContext().directive, blockDir.source);
     break;
   case llvm::acc::Directive::ACCD_data:
     // Restriction - line 1249-1250
     CheckRequireAtLeastOneOf();
+    // Restriction is not formally in the specification but all compilers emit
+    // an error and it is likely to be omitted from the spec.
+    CheckNoBranching(block, GetContext().directive, blockDir.source);
     break;
   case llvm::acc::Directive::ACCD_host_data:
     // Restriction - line 1746
     CheckRequireAtLeastOneOf();
     break;
   default:
     break;
   }

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

program openacc_clause_validity
   do i = 1, N
     a(i) = 3.14
     if(i == N-1) THEN
       stop 999 ! no error
     end if
   end do
   !$acc end serial
 
+
+  !$acc data create(a)
+
+  !ERROR: RETURN statement is not allowed in a DATA construct
+  if (size(a) == 10) return
+
+  !$acc end data
+
 end program openacc_clause_validity