[clang] [llvm] Add macro to suppress -Wunnecessary-virtual-specifier (PR #139614)

[Devon Loehr via cfe-commits]

DKLoehr wrote:

> I thought we'd have more anchors, honestly. Maybe these are just the overriding ones.

Yes, these are just the ones that triggered the warning (i.e. those in `final` classes that didn't inherit them from an ancestor).

https://github.com/llvm/llvm-project/pull/139614