[clang] [Clang] disallow # operators in attribute argument lists (PR #147308)

[Erich Keane via cfe-commits]

================
@@ -124,6 +124,11 @@ Improvements to Clang's diagnostics
   Added a new warning in this group for the case where the attribute is missing/implicit on
   an override of a virtual method.
 
+- Clang rejects the ``#`` and ``##`` preprocessor tokens in an attribute
+  argument list in C++. The operators can be used in macro replacement lists
+  with the usual preprocessor semantics. What is rejected are non-preprocessor
----------------
erichkeane wrote:

```suggestion
  with the usual preprocessor semantics, however non-preprocessor use of tokens are now rejected in C++.  Compilation in C mode is unchanged, and still permits these tokens to be used.
```

https://github.com/llvm/llvm-project/pull/147308