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

[Mariya Podchishchaeva via cfe-commits]

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@@ -1737,7 +1737,18 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
       Diag(getLastFPEvalPragmaLocation(), diag::note_pragma_entered_here);
     }
   } else if (II == Ident__COUNTER__) {
-    // __COUNTER__ expands to a simple numeric value.
+    Diag(Tok.getLocation(),
+         getLangOpts().C2y ? diag::warn_counter : diag::ext_counter);
+    // __COUNTER__ expands to a simple numeric value that must be less than
+    // 2147483647.
+    if (CounterValue > 2147483647) {
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Fznamznon wrote:

Maybe makes sense to put `2147483647` into a constant somewhere?

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