[flang][openacc] Fix clause restriction for host_data directive

[clementval]

Summary

Restriction on clauses for the HOST_DATA directive were not fully correct.
This patch fixes the situation. The if and if_present clauses are allowed
only once.

[SouraVX]

LGTM! Thanks!

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

program openacc_host_data_validity
   !$acc end host_data
 
   !$acc host_data use_device(aa) if(ifCondition)
   !$acc end host_data
 
   !$acc host_data use_device(aa, bb) if_present
   !$acc end host_data
 
+  !ERROR: At most one IF_PRESENT clause can appear on the HOST_DATA directive
+  !$acc host_data use_device(aa, bb) if_present if_present
+  !$acc end host_data
+
   !$acc host_data use_device(aa, bb) if(.true.) if_present
   !$acc end host_data
 
+  !ERROR: At most one IF clause can appear on the HOST_DATA directive
   !$acc host_data use_device(aa, bb) if(.true.) if(ifCondition)
   !$acc end host_data
 
 end program openacc_host_data_validity

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

let requiredClauses = [
     VersionedClause<ACCC_Copyout>,
     VersionedClause<ACCC_Delete>,
     VersionedClause<ACCC_Detach>
   ];
 }
 
 // 2.8
 def ACC_HostData : Directive<"host_data"> {
-  let allowedClauses = [
+  let allowedOnceClauses = [
     VersionedClause<ACCC_If>,
     VersionedClause<ACCC_IfPresent>
   ];
   let requiredClauses = [
     VersionedClause<ACCC_UseDevice>
   ];
 }