OwningArrayRef is being removed upstream, per
https://github.com/llvm/llvm-project/pull/169126. This replaces uses
with `SmallVector`.
I've also made a separate commit which does init changes; these aren't
strictly necessary, but I added to make it a little more idiomatic in
spots.
---------
Co-authored-by: Dana Jansens <danakj@orodu.net>