[flang][OpenMP] Parser support for the unroll construct (5.1)

added parser support for the unroll construct

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D138229

flang/lib/Parser/unparse.cpp

@@ -2153,6 +2153,9 @@ public:
     case llvm::omp::Directive::OMPD_tile:
       Word("TILE ");
       break;
+    case llvm::omp::Directive::OMPD_unroll:
+      Word("UNROLL ");
+      break;
     default:
       break;
     }