[clang] [CIR] Add CIR vtable attribute (PR #154415)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> Is this also meant to cover VTTs?

As we discussed earlier, this isn't used for VTTs. Currently, the incubator emits VTTs as a constant array of global view pointers to vtables. We may want to consider a wrapper attribute similar to this for VTTs.

https://github.com/llvm/llvm-project/pull/154415