fix incorrect static parameter definedness check (#49097)
We use the specialized signature of a method for accurate analysis of
whether a static parameter is constrained or not. However, it turns out
that we can only use it when it doesn't contain any free type variables
(which it sometimes does, e.g., when the inference entry is given a
signature with a free type variable). In such cases, we should use the
method signature, which, by design, never contains any free type
variables.
