[flang][OpenMP] Remove pessimizing move introduced in 90f45a1

This unbreaks the builders that diagnose this situation:

  error: moving a temporary object prevents copy elision [-Werror,-Wpess
imizing-move]
    341 |   static OmpClauseList empty{std::move(decltype(OmpClauseList:
:v){})};
        |                              ^

Author: kparzysz

flang/lib/Parser/parse-tree.cpp

@@ -338,7 +338,7 @@ llvm::omp::Clause OmpClause::Id() const {
 }
 
 const OmpClauseList &OmpDirectiveSpecification::Clauses() const {
-  static OmpClauseList empty{std::move(decltype(OmpClauseList::v){})};
+  static OmpClauseList empty{decltype(OmpClauseList::v){}};
   if (auto &clauses = std::get<std::optional<OmpClauseList>>(t)) {
     return *clauses;
   }