[llvm] [llubi] Add support for captures (PR #201170)

[Ralf Jung via llvm-commits]

RalfJung wrote:

Also could you summarize what the semantics are now for the various provenance-related `captures` cases?
We may even want to update the LangRef to make that more explicit (`nofree` already very explicitly talks about what happens to the provenance, that would be a good example to follow).

https://github.com/llvm/llvm-project/pull/201170