[clang] [llvm] Enable unnecessary-virtual-specifier by default (PR #133265)

[Hans Wennborg via cfe-commits]

zmodem wrote:

> I think moving it to -Wextra may be a more palatable approach. What do others think?

Sounds good to me, and I think James makes a good argument.

Does that actually help LLVM and libc++ though? I think at least LLVM does enable -Wextra.

https://github.com/llvm/llvm-project/pull/133265