[clang] [C23] Implement N3018: The constexpr specifier for object definitions (PR #73099)

[Aaron Ballman via cfe-commits]

================
@@ -2932,6 +2932,22 @@ def warn_private_extern : Warning<
 def note_private_extern : Note<
   "use __attribute__((visibility(\"hidden\"))) attribute instead">;
 
+// C23 constexpr
+def err_c23_thread_local_constexpr : Error<
+  "thread-local storage is not allowed with constexpr">;
+def err_c23_extern_constexpr : Error<
+  "extern specifier is not allowed with constexpr">;
+def err_c23_constexpr_not_variable : Error<
+  "constexpr is only allowed in variable declarations">;
----------------
AaronBallman wrote:

```suggestion
  "'constexpr' can only be used in variable declarations">;
```

https://github.com/llvm/llvm-project/pull/73099