swift234

[closure-spec] Refactor closure spec so that we can in a subsequent commit handle both partial_apply and thin_to_thick_function.

Since this is just a refactoring, it should be NFC.

This is to fix a dumb bug where we will propagate closures if the
closure captures variables (i.e. partial_apply), but we will not
propagate closures that do not capture any values (i.e.
thin_to_thick_function).

  a.map {
    return $0 + v
  }

But this will not be:

  a.map {
    return $0 + 1
  }

This will also give us the flexibility in the future to support the
propagation of function pointers (at which point we should probably
rename this pass).

Swift SVN r24759