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

[Devon Loehr via cfe-commits]

DKLoehr wrote:

I've only had ad-hoc discussions on the matter, but everyone I've talked to seems to fall in the latter camp. As mentioned in #138741, I didn't see any other project with a similar policy among chromium and all of its dependencies. I'd have no problem if we decided to do away with it altogether, but I don't know what that involves. It's worth noting that the instance in Sema.h is even more unusual, per the comment above it.

In the meantime, this gets us back to the status quo, and lets us use the warning to detect truly-accidental `virtual` specifiers.

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