[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#124391)

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 DeclOrIterator to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaCodeComplete.cpp

@@ -678,7 +678,7 @@ class ResultBuilder::ShadowMapEntry::iterator {
   }*/
 
   reference operator*() const {
-    if (const NamedDecl *ND = DeclOrIterator.dyn_cast<const NamedDecl *>())
+    if (const NamedDecl *ND = dyn_cast<const NamedDecl *>(DeclOrIterator))
       return reference(ND, SingleDeclIndex);
 
     return *cast<const DeclIndexPair *>(DeclOrIterator);