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`.

compiler/rustc_pattern_analysis/tests/intersection.rs

@@ -14,7 +14,8 @@ fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<Vec<usize>> {
     let arms: Vec<_> =
         patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
     let report =
-        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, None).unwrap();
+        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, usize::MAX)
+            .unwrap();
     report.arm_intersections.into_iter().map(|bitset| bitset.iter().collect()).collect()
 }