[ty] Make `infer_method_information` less confusing (#20740)
## Summary
`infer_method_information` was previously calling
`ClassLiteral::to_class_type`, which uses the default-specialization of
a generic class. This specialized `ClassType` was later only used if the
class was non-generic, making the specialization irrelevant. The
implementation was still a bit confusing, so this PR proposes a way to
avoid turning the class literal into a `ClassType`.
