rustc_infer: remove InferCtxt::closure_sig as the FnSig is always shallowly known.
That is, `ClosureSubsts` is always created (in `rustc_typeck::check::closure`) with a `FnSig`, as the number of inputs is known, even if they might all have inference types.
The only useful thing `InferCtxt::closure_sig` was doing is resolving an inference variable used just to get the `ty::FnPtr` containing that `FnSig` into `ClosureSubsts`.
The ideal way to solve this would be to add a constructor for `ClosureSubsts`, that combines the parent `Substs`, the closure kind, the signature, and capture types together, but for now I've went with resolving the inference types just after unifying them with the real types.
r? @nikomatsakis
