Types and specifically "mentioned items" take up a lot of space in Rust metadata files

https://github.com/rust-lang/rust/pull/122568 was a significant size regression for Rust metadata. The extra information that is stored in MIR bodies now is a bunch of Ty<'tcx>, so in memory I think this is actually not that much. But it seems like in our metadata format this takes up quite a bit of space. This not only makes the library files bigger, it also accounts for a large fraction (I think even the majority) of the compile-time regression from that PR.

I don't know if there's something that can be done to improve this -- either by storing different information in mentioned_items that needs less space on disk, or by representing Ty<'tcx> more efficiently on disk. One drastic option would be to "intern" types in the on-disk format, i.e. have one global table of types that everything else just indexes into. That would certainly save space when the same type appears multiple times. I don't know if that is what happens here though.

Start by comparing Rust PR 122568 with the metadata produced for MIR bodies, focusing on the mentioned_items data and serialized Ty<'tcx> values. Investigate whether repeated types or mentioned items dominate the on-disk size and compile-time regression. Done means reducing the metadata size and associated compile-time impact without losing the required information.