[clang] [C2y] Support WG14 N3457, the __COUNTER__ macro (PR #162662)

[Timm Baeder via cfe-commits]

================
@@ -90,6 +90,14 @@ def err_unterminated___pragma : Error<"missing terminating ')' character">;
 
 def err_conflict_marker : Error<"version control conflict marker in file">;
 
+def err_counter_overflow : Error<
+  "'__COUNTER__' value cannot exceed 2147483647">;
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tbaederr wrote:

```suggestion
  "'__COUNTER__' value cannot exceed 2'147'483'647">;
```
? Or is that not useful here because this is only emitted in C?

https://github.com/llvm/llvm-project/pull/162662