rustc_metadata: Encode/decode some `LazyArrays` without an `Option`
and a couple of related changes, see individual commits.
Addresses comments in https://github.com/rust-lang/rust/pull/107166#discussion_r1083417124 and https://github.com/rust-lang/rust/pull/107166#discussion_r1083768417, cc `@cjgillot` `@oli-obk.`