Method invocation should preserve generic annotation on `Self`

[grievejia]

MInimal repro:

class A:
    def m[S: A](self: S) -> S:  # The bound of `A` on `S` is optional
        return self

def g[T: A](a: T) -> T:
    return a.m()

Expected: no error Actual: Returned type A is not assignable to declared return type TypeVar[T] [bad-return]

When calculating the type of a.m(), we seem to be pinning the type of a to A immediately -- that's incorrect. The type should be T (where T may be either A or one of A's subclass).

[yangdanny97]

https://github.com/facebook/pyrefly/issues/119

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