[clang] [Clang] Fix -Wunused-private-field false negative with defaulted comparison operators (PR #116871)

[Mészáros Gergely via cfe-commits]

https://github.com/Maetveis approved this pull request.

LGTM Thanks!. Can you add a note in the clang release notes here: https://github.com/llvm/llvm-project/blob/d8a1c6d72739c99f4b52a84e907b96b685b44c5e/clang/docs/ReleaseNotes.rst?plain=1#L559

I think this should be good for the text:
```rst
- Fixed a bug where Clang would not emit ``-Wunused-private-field`` warnings when an unrelated class 
  defined a defaulted comparison operator (#GH116270).

  .. code-block:: c++

    class A {
    private:
      int a; // warning: private field 'a' is not used, no diagnostic previously
    };

    class C {
      bool operator==(const C&) = default;
    };

```

(If you want to build the clang documentation to check locally, you can follow https://github.com/llvm/llvm-project/blob/main/llvm/docs/README.txt#L11 and replace `docs-llvm-html` with `docs-clang-html`.)

https://github.com/llvm/llvm-project/pull/116871