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

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -verify=ext -std=c23 -pedantic %s
+// RUN: %clang_cc1 -verify=pre -std=c2y -pedantic -Wpre-c2y-compat %s
+
+/* WG14 N3457: Clang 22
+ * The __COUNTER__ predefined macro
+ *
+ * This predefined macro was supported as an extension in earlier versions of
+ * Clang, but the required diagnostics for the limits were not added until 22.
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AaronBallman wrote:

Sure! It will say "C2y extension", but that's expected.

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