[clang] [clang][PAC] ptrauth_qualifier must be considered a feature (PR #153291)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Right, so in macro expansion we have

```cpp
  // If the use of an extension results in an error diagnostic, extensions are
  // effectively unavailable, so just return false here.
  if (PP.getDiagnostics().getExtensionHandlingBehavior() >=
      diag::Severity::Error)
    return false;
```

But if you have `-Wno-<some-extension-warning>` the use of the extension would in principle not generate an warning, and so this would be "ok" and this path should not have been taken.

https://github.com/llvm/llvm-project/pull/153291