Rollup merge of #121695 - oli-obk:split_ty_utils, r=compiler-errors

Split rustc_type_ir to avoid rustc_ast from depending on it

unblocks #121576

and resolves a FIXME in `rustc_ast`'s `Cargo.toml`

The new crate is tiny, but it will get bigger in #121576

Author: GuillaumeGomez