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/common/mod.rs

@@ -124,7 +124,7 @@ pub fn compute_match_usefulness<'p>(
     arms: &[MatchArm<'p, Cx>],
     ty: Ty,
     scrut_validity: PlaceValidity,
-    complexity_limit: Option<usize>,
+    complexity_limit: usize,
 ) -> Result<UsefulnessReport<'p, Cx>, ()> {
     init_tracing();
     rustc_pattern_analysis::usefulness::compute_match_usefulness(