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

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 CO to be nonnull.

Author: kazutakahirata

clang/lib/AST/JSONNodeDumper.cpp

@@ -1537,9 +1537,9 @@ void JSONNodeDumper::VisitExprWithCleanups(const ExprWithCleanups *EWC) {
   if (EWC->getNumObjects()) {
     JOS.attributeArray("cleanups", [this, EWC] {
       for (const ExprWithCleanups::CleanupObject &CO : EWC->getObjects())
-        if (auto *BD = CO.dyn_cast<BlockDecl *>()) {
+        if (auto *BD = dyn_cast<BlockDecl *>(CO)) {
           JOS.value(createBareDeclRef(BD));
-        } else if (auto *CLE = CO.dyn_cast<CompoundLiteralExpr *>()) {
+        } else if (auto *CLE = dyn_cast<CompoundLiteralExpr *>(CO)) {
           llvm::json::Object Obj;
           Obj["id"] = createPointerRepresentation(CLE);
           Obj["kind"] = CLE->getStmtClassName();