The IdBindingInfo field of ATcId serves two purposes

- to control generalisation when we have -XMonoLocalBinds
- to check for floatability when dealing with (static e)

These are related, but not the same, and they'd becomme confused.
Trac #13804 showed this up via an example like this:

  f periph = let sr :: forall a. [a] -> [a]
                 sr = if periph then reverse else id

                 sr2 = sr
                 -- The question: is sr2 generalised?
                 -- It should be, because sr has a type sig
                 -- even though it has periph free
             in
             (sr2 [True], sr2 "c")

Here sr2 should be generalised, despite the free var 'periph'
in 'sr' because 'sr' has a closed type signature.

I documented all this very carefully this time, in TcRnTypes:
  Note [Meaning of IdBindingInfo]
  Note [Bindings with closed types: ClosedTypeId]