Add pattern_complexity_limit to Limits.

It's similar to the other limits, e.g. obtained via `get_limit`. So it
makes sense to handle it consistently with the other limits. We now use
`Limit`/`usize` in most places instead of `Option<usize>`, so we use
`Limit::new(usize::MAX)`/`usize::MAX` to emulate how `None` used to work.

The commit also adds `Limit::unlimited`.

Author: nnethercote

crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs

@@ -95,7 +95,7 @@ impl<'db> MatchCheckCtx<'db> {
 
         let place_validity = PlaceValidity::from_bool(known_valid_scrutinee.unwrap_or(true));
         // Measured to take ~100ms on modern hardware.
-        let complexity_limit = Some(500000);
+        let complexity_limit = 500000;
         compute_match_usefulness(self, arms, scrut_ty, place_validity, complexity_limit)
     }