[Clang][OpenMP] Add static version of getSingleClause<ClauseT>. NFC.

[Meinersbur]

Summary

The current method getSingleClause requires an instance of OMPExecutableDirective to be called. Introduce a static version taking a list of clauses as argument instead that can be used during parsing/Sema before any OMPExecutableDirective has been created.

This is the same approach as taken for getClausesOfKind for getting more more than a single clause of a type which also has a method and static version. NFC patch extracted out of D99459 by request.

[Meinersbur]

Add context

[ABataev]

LG

clang/include/clang/AST/StmtOpenMP.h

public:
   }
 
   /// Gets a single clause of the specified kind associated with the
   /// current directive iff there is only one clause of this kind (and assertion
   /// is fired if there is more than one clause is associated with the
   /// directive). Returns nullptr if no clause of this kind is associated with
   /// the directive.
   template <typename SpecificClause>
-  const SpecificClause *getSingleClause() const {
-    auto Clauses = getClausesOfKind<SpecificClause>();
+  static const SpecificClause *getSingleClause(ArrayRef<OMPClause *> Clauses) {
+    auto ClausesOfKind = getClausesOfKind<SpecificClause>(Clauses);
 
-    if (Clauses.begin() != Clauses.end()) {
-      assert(std::next(Clauses.begin()) == Clauses.end() &&
+    if (ClausesOfKind.begin() != ClausesOfKind.end()) {
+      assert(std::next(ClausesOfKind.begin()) == ClausesOfKind.end() &&
              "There are at least 2 clauses of the specified kind");
-      return *Clauses.begin();
+      return *ClausesOfKind.begin();
     }
     return nullptr;
   }
 
+  template <typename SpecificClause>
+  const SpecificClause *getSingleClause() const {
+    return getSingleClause<SpecificClause>(clauses());
+  }
+
   /// Returns true if the current directive has one or more clauses of a
   /// specific kind.
   template <typename SpecificClause>
   bool hasClausesOfKind() const {
     auto Clauses = getClausesOfKind<SpecificClause>();
     return Clauses.begin() != Clauses.end();
   }