[clang] fix bug that undefined internal is a warning only for -pedantic-errors (PR #98016)

[Aaron Ballman via cfe-commits]

================
@@ -554,6 +554,9 @@ Attribute Changes in Clang
 
 Improvements to Clang's diagnostics
 -----------------------------------
+- Clang now emits an error instead of a warning for `undefined_internal`
----------------
AaronBallman wrote:

```suggestion
- Clang now emits an error instead of a warning for ``-Wundefined-internal``
```

https://github.com/llvm/llvm-project/pull/98016