[clang] [Clang] Mark WG14 N2939 (Identifier Syntax Fixes) as available in Clang 15 (PR #74666)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,83 @@
+// RUN: %clang_cc1 -x c -std=c2x -fsyntax-only -verify %s
+// RUN: %clang_cc1 -x c -std=c2x -E -DPP_ONLY=1 %s | FileCheck %s --strict-whitespace
+
+/* WG14 N2863: Clang 15
----------------
AaronBallman wrote:

```suggestion
/* WG14 N2836: Clang 15
```

https://github.com/llvm/llvm-project/pull/74666