This reverts commit adf9b5fcd1de43eaf0a779e10612caee8b47bede, reversing
changes made to af1ca153d4aed5ffe22445273aa388a8d3f8f4ae.
Reverting due to <https://github.com/rust-lang/rust/issues/133606>.
Functions currently can't have mappings in multiple files, and if that ever
changes (e.g. to properly support expansion regions), this code will need to be
completely overhauled anyway.