[clang-tools-extra] [clang-tidy] Add detection of decltype(nullptr) => std::nullptr_t to … (PR #224591)

[Baranov Victor via cfe-commits]

================
@@ -51,6 +61,11 @@ Options
    ``NULL``. By default this check will only replace the ``NULL`` macro and will
    skip any similar user-defined macros.
 
+.. option:: NullptrCStddef
+
+   Boolean controlling wether we should replace ``decltype(nullptr)`` with the
+   type ``std::nullptr_t`` from ``cstddef``. Defaults to ``true``.
----------------
vbvictor wrote:

```suggestion
   type ``std::nullptr_t`` from ``<cstddef>``. Defaults to ``true``.
```

https://github.com/llvm/llvm-project/pull/224591