Add codegen test for `Request::provide_*`
Codegen before & after https://github.com/rust-lang/rust/pull/126242: https://gist.github.com/slanterns/3789ee36f59ed834e1a6bd4677b68ed4.
Also adjust an outdated comment since `tag_id` is no longer attached to `TaggedOption` via `Erased`, but stored next to it in `Tagged` under the new implementation.
My first time writing FileCheck xD. Correct me if there is anything that should be amended.
r? libs