ZJIT: Make hir::types::Class not final and have it include metaclasses

Every class boots with a metaclass, and all metaclasses are subclasses
of Class, so `types::Class` has no business in `ExactBitsAndClass`.
In fact, we should never see an object whose RBasic::class is exactly
rb_cClass because classes get a metaclass on boot. So there is no
ClassExact type.

This fixes the side exits on getivar-module.rb that were introduced in
a8f3c34 ("ZJIT: Add missing guard
on ivar access on T_{DATA,CLASS,MODULE}"). The `GuardType v, Class` checked
for exactly rb_cClass and never passed.

Author: XrXr

zjit/src/hir_type/gen_hir_type.rb

@@ -89,7 +89,7 @@ def base_type name, c_name: nil
   [type, exact]
 end
 
-# Define a new type that cannot be subclassed.
+# Define a new type that has no subclasses and cannot be subclassed.
 # If c_name is given, mark the rb_cXYZ object as equivalent to this type.
 def final_type name, base: $object, c_name: nil
   if c_name
@@ -109,7 +109,9 @@ def final_type name, base: $object, c_name: nil
 base_type "Set", c_name: "rb_cSet"
 base_type "Regexp", c_name: "rb_cRegexp"
 module_class, _ = base_type "Module", c_name: "rb_cModule"
-class_ = final_type "Class", base: module_class, c_name: "rb_cClass"
+# Class cannot be subclassed by doing `class Sub < Class`,
+# but every metaclass is a subclass of `Class`. It's not final.
+module_class.subtype "Class"
 
 numeric, _ = base_type "Numeric", c_name: "rb_cNumeric"