[clang] Warn about virtual methods in `final` classes (PR #131188)

[Hans Wennborg via cfe-commits]

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@@ -2706,6 +2706,9 @@ def warn_final_dtor_non_final_class : Warning<
   InGroup<FinalDtorNonFinalClass>;
 def note_final_dtor_non_final_class_silence : Note<
   "mark %0 as '%select{final|sealed}1' to silence this warning">;
+def warn_unnecessary_virtual_specifier : Warning<
+  "virtual method %0 is inside a 'final' class and can never be overridden">,
+  InGroup<WarnUnnecessaryVirtualSpecifier>, DefaultIgnore;
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zmodem wrote:

Instead of DefaultIgnore, could we turn this on by default? I think it should be pretty high fidelity. Or at least we could put it in -Wall or -Wextra?

https://github.com/llvm/llvm-project/pull/131188