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

[James Y Knight via llvm-commits]

jyknight wrote:

LLVM's intentional usage is either to reduce redundant vtable emission via ensuring there is a key method, OR, to intentionally create a useless vtable, in order to reduce redundant debuginfo emission (it can be emitted only in the vtable's TU when there's a vtable.) I expect both are likely to be rather unusual.

I'd say it's highly likely that _most_ code where this diagnostic triggers is simply because "virtual" was added to a function which simply doesn't need it, and the solution is to remove "virtual". This diagnostic can definitely be useful -- it likely indicates an issue in the code. I just don't think the issue it identifies is _that important_ as to warrant being on by default, such that we'd emit this even on a project with has not enabled any warning flags.

E.g. as comparison, is this really more important than `-Wall` diagnostics like "-Wdelete-non-virtual-dtor" ("warning: delete called on non-final <Foo> that has virtual functions but non-virtual destructor"), for example? I'd say definitely not.

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