[clang] [Clang] [NFC] Introduce a helper for emitting compatibility diagnostics (PR #132348)

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@@ -178,6 +191,11 @@ multiclass CXXCompat<
                                      "CXX98Compat",
                                      "CXXPre"#std_ver#"Compat"))>,
         DefaultIgnore;
+
+    def : CompatWarningId<
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Sirraide wrote:

I’m still figuring out how to integrate the C compatibility warnings; for now this is just for C++, but I think we can definitely extend this later

https://github.com/llvm/llvm-project/pull/132348