[flang] Fix a warning

This patch fixes:

  flang/lib/Lower/OpenMP/OpenMP.cpp:2346:14: error: unused variable
  'origDirective' [-Werror,-Wunused-variable]

Author: kazutakahirata

flang/lib/Lower/OpenMP/OpenMP.cpp

@@ -2352,6 +2352,7 @@ genOMP(Fortran::lower::AbstractConverter &converter,
 
   assert(llvm::omp::blockConstructSet.test(origDirective) &&
          "Expected block construct");
+  (void)origDirective;
 
   for (const Clause &clause : clauses) {
     mlir::Location clauseLocation = converter.genLocation(clause.source);