[red-knot] optimize is_subtype_of for literals (#17394)

## Summary

Allows us to establish that two literals do not have a subtype
relationship with each other, without having to fallback to a typeshed
Instance type, which is comparatively slow.

Improves the performance of the many-string-literals union benchmark by
5x.

## Test Plan

`cargo test -p red_knot_python_semantic` and `cargo bench --bench
red_knot`.

crates/red_knot_python_semantic/src/types.rs

@@ -886,6 +886,27 @@ impl<'db> Type<'db> {
                 target.is_equivalent_to(db, Type::object(db))
             }
 
+            // No literal type is a subtype of any other literal type, unless they are the same
+            // type (which is handled above). This case is not necessary from a correctness
+            // perspective (the fallback cases below will handle it correctly), but it is important
+            // for performance of simplifying large unions of literal types.
+            (
+                Type::StringLiteral(_)
+                | Type::IntLiteral(_)
+                | Type::BytesLiteral(_)
+                | Type::ClassLiteral(_)
+                | Type::FunctionLiteral(_)
+                | Type::ModuleLiteral(_)
+                | Type::SliceLiteral(_),
+                Type::StringLiteral(_)
+                | Type::IntLiteral(_)
+                | Type::BytesLiteral(_)
+                | Type::ClassLiteral(_)
+                | Type::FunctionLiteral(_)
+                | Type::ModuleLiteral(_)
+                | Type::SliceLiteral(_),
+            ) => false,
+
             // All `StringLiteral` types are a subtype of `LiteralString`.
             (Type::StringLiteral(_), Type::LiteralString) => true,