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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @DKLoehr @AaronBallman Did you see the revert?

Thank you for the ping, I did not see the revert. Thanks for catching that and sorry for the disruption!

Given that this disrupts both LLVM and libc++, I think that's plenty of signal that "on by default" isn't really the best approach here. I think moving it to `-Wextra` may be a more palatable approach. What do others think?

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