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_session/src/session.rs

@@ -67,6 +67,11 @@ impl Limit {
         Limit(value)
     }
 
+    /// Create a new unlimited limit.
+    pub fn unlimited() -> Self {
+        Limit(usize::MAX)
+    }
+
     /// Check that `value` is within the limit. Ensures that the same comparisons are used
     /// throughout the compiler, as mismatches can cause ICEs, see #72540.
     #[inline]
@@ -119,6 +124,8 @@ pub struct Limits {
     pub move_size_limit: Limit,
     /// The maximum length of types during monomorphization.
     pub type_length_limit: Limit,
+    /// The maximum pattern complexity allowed (internal only).
+    pub pattern_complexity_limit: Limit,
 }
 
 pub struct CompilerIO {