[clang-tools-extra] [clang-tidy][NFC] Apply llvm-redundant-casting check (PR #214024)

[Yanzuo Liu via cfe-commits]

================
@@ -69,8 +69,7 @@ struct NotLengthExprForStringNode {
           return true;
         }
 
-        if (const auto *OnNode =
-                dyn_cast<Expr>(MemberCallNode->getImplicitObjectArgument())) {
+        if (const auto *OnNode = MemberCallNode->getImplicitObjectArgument()) {
----------------
zwuis wrote:

```suggestion
        if (const Expr *OnNode = MemberCallNode->getImplicitObjectArgument()) {
```


https://github.com/llvm/llvm-project/pull/214024