rustc_session: allow overriding lint level of individual lints from a group
Fixes#58211 and fixesrust-lang/rust-clippy#4778 and fixesrust-lang/rust-clippy#4091
Instead of hard-coding the lint level preferences (from lowest to highest precedence: `lint::Allow -> lint::Warn -> lint::Deny -> lint::Forbid`), the position of the argument in the command line gets taken into account.
Examples:
1. Passing `-D unused -A unused-variables` denies everything in the lint group `unused` **except** `unused-variables` which is explicitly allowed.
1. Passing `-A unused-variables -D unused` denies everything in the lint group `unused` **including** `unused-variables` since the allow is specified before the deny (and therefore overridden by the deny).
This matches the behavior that is already being used when specifying `allow`/`deny` in the source code.
5e7af4669f