[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#122651)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect Ptr to be nonnull.

Author: kazutakahirata

clang/include/clang/AST/DeclBase.h

@@ -1334,7 +1334,7 @@ class DeclListNode {
 
     reference operator*() const {
       assert(Ptr && "dereferencing end() iterator");
-      if (DeclListNode *CurNode = Ptr.dyn_cast<DeclListNode*>())
+      if (DeclListNode *CurNode = dyn_cast<DeclListNode *>(Ptr))
         return CurNode->D;
       return cast<NamedDecl *>(Ptr);
     }
@@ -1344,7 +1344,7 @@ class DeclListNode {
     inline iterator &operator++() { // ++It
       assert(!Ptr.isNull() && "Advancing empty iterator");
 
-      if (DeclListNode *CurNode = Ptr.dyn_cast<DeclListNode*>())
+      if (DeclListNode *CurNode = dyn_cast<DeclListNode *>(Ptr))
         Ptr = CurNode->Rest;
       else
         Ptr = nullptr;