Add a `parse_sess_created` callback to `rustc_interface::interface::Config`

imo this is fine, but we should look into figuring out the story around adding custom things to the depinfo in general. This seems like a larger topic (that may want to include proc macros and similar).

I think we should merge this as it is and then (once clippy is synced into the rustc repo) consider doing a change there that adds a parse_sess_created callback that allows you to modify the session right after it was created.

Originally posted by @oli-obk in rust-lang/rust-clippy#6834 (comment)

cc @flip1995

@rustbot label A-clippy

[jyn514]

cc #82763 (which has the benefit of being in-tree and can just hardcode the things it wants to do in rustc_session)

once clippy is synced into the rustc repo

It has been synced now.
@rustbot claim