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This turns on the unnecessary-virtual-specifier warning in general, but disables it when building LLVM. It also tweaks the warning description to be slightly more accurate. Background: I've been working on cleaning up this warning in two codebases: LLVM and chromium (plus its dependencies). The chromium cleanup has been straightforward. Git archaeology shows that there are two reasons for the warnings: classes to which `final` was added after they were initially committed, and classes with virtual destructors that nobody remarks on. Presumably the latter case is because people are just very used to destructors being virtual. The LLVM cleanup was more surprising: I discovered that we have an [old policy](https://llvm.org/docs/CodingStandards.html#provide-a-virtual-method-anchor-for-classes-in-headers) about including out-of-line virtual functions in every class with a vtable, even `final` ones. This means our codebase has many virtual "anchor" functions which do nothing except control where the vtable is emitted, and which trigger the warning. I looked into alternatives to satisfy the policy, such as using destructors instead of introducing a new function, but it wasn't clear if they had larger implications. Overall, it seems like the warning is genuinely useful in most codebases (evidenced by chromium and its dependencies), and LLVM is an unusual case. Therefore we should enable the warning by default, and turn it off only for LLVM builds.
See docs/CMake.html for instructions on how to build LLVM with CMake.