Auto merge of #86970 - inquisitivecrystal:force-warn, r=davidtwco

Auto merge of #86970 - inquisitivecrystal:force-warn, r=davidtwco

Make `--force-warns` a normal lint level option

Now that `ForceWarn` is a lint level, there's no reason `--force-warns` should be treated differently from other options that set lint levels. This merges the `ForceWarn` handling in with the other lint level command line options. It also unifies all of the relevant selection logic in `compiler/rustc_lint/src/levels.rs`, rather than having some of it weirdly elsewhere.

Fixes #86958, which arose from the special-cased handling of `ForceWarn` having had an error in it.