[clang] [llvm] Add unnecessary-virtual-specifier to -Wextra (PR #138741)

[Nikolas Klauser via llvm-commits]

philnik777 wrote:

This should probably work. To make sure you can make a simple change in the `libcxx/` subdirectory to trigger the libc++ pre-commit CI (which should of course be removed again before actually committing).

https://github.com/llvm/llvm-project/pull/138741