[clang] [C23][Parser] Diagnostic for attribute declaration where statement is required (PR #146224)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c2x -verify %s
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AaronBallman wrote:

```suggestion
// RUN: %clang_cc1 -fsyntax-only -std=c23 -verify %s
```
Maybe rename the file to be `c23` instead of `c2x` as well?

https://github.com/llvm/llvm-project/pull/146224