[flang][OpenMP] Remove std::variant with single alternative, NFC

Author: kparzysz

flang/include/flang/Parser/parse-tree.h

@@ -3695,9 +3695,8 @@ struct OmpMapClause {
 
 // 2.9.5 order-clause -> ORDER ([order-modifier :]concurrent)
 struct OmpOrderModifier {
-  UNION_CLASS_BOILERPLATE(OmpOrderModifier);
   ENUM_CLASS(Kind, Reproducible, Unconstrained)
-  std::variant<Kind> u;
+  WRAPPER_CLASS_BOILERPLATE(OmpOrderModifier, Kind);
 };
 
 struct OmpOrderClause {

flang/lib/Lower/OpenMP/Clauses.cpp

@@ -1092,8 +1092,7 @@ Order make(const parser::OmpClause::Order &inp,
   auto &t1 = std::get<wrapped::Type>(inp.v.t);
 
   auto convert3 = [&](const parser::OmpOrderModifier &s) {
-    return Fortran::common::visit(
-        [&](parser::OmpOrderModifier::Kind k) { return convert1(k); }, s.u);
+    return convert1(s.v);
   };
   return Order{
       {/*OrderModifier=*/maybeApply(convert3, t0), /*Ordering=*/convert2(t1)}};