[flang] Fix a warning

This patch fixes:

  flang/lib/Lower/OpenMP/OpenMP.cpp:599:15: error: unused variable
  'ompEval' [-Werror,-Wunused-variable]

Author: kazutakahirata

flang/lib/Lower/OpenMP/OpenMP.cpp

@@ -600,6 +600,7 @@ static void processHostEvalClauses(lower::AbstractConverter &converter,
   assert(ompEval &&
          llvm::omp::allTargetSet.test(extractOmpDirective(*ompEval)) &&
          "expected TARGET construct evaluation");
+  (void)ompEval;
 
   // Use the whole list of clauses passed to the construct here, rather than the
   // ones only applied to omp.target.