[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#125157)

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 NodeOrVector to be nonnull.

Author: kazutakahirata

clang/lib/AST/ParentMapContext.cpp

@@ -395,7 +395,7 @@ class ParentMapContext::ParentMap::ASTVisitor
       if (!isa<ParentVector *>(NodeOrVector)) {
         auto *Vector = new ParentVector(
             1, getSingleDynTypedNodeFromParentMap(NodeOrVector));
-        delete NodeOrVector.template dyn_cast<DynTypedNode *>();
+        delete dyn_cast<DynTypedNode *>(NodeOrVector);
         NodeOrVector = Vector;
       }