When we calculate which typevars are inferable in a generic context, the
result might include more than the typevars bound by the generic
context. The canonical example is a generic method of a generic class:
```py
class C[A]:
def method[T](self, t: T): ...
```
Here, the inferable typevar set of `method` contains `Self` and `T`, as
you'd expect. (Those are the typevars bound by the method.) But it also
contains `A@C`, since the implicit `Self` typevar is defined as `Self:
C[A]`. That means when we call `method`, we need to mark `A@C` as
inferable, so that we can determine the correct mapping for `A@C` at the
call site.
Fixes https://github.com/astral-sh/ty/issues/1874
cbfecfaf41