Fixes #15925.

This issue is caused by a bug in mapInferred during CC Setup. In this function we convert all top-level non-dependent functions to its dependent counterpart. However, it does not correctly handle the case when the type is an alias to function types. The problematic case in the pattern matching:

case tp @ AppliedType(tycon, args) =>
  val tycon1 = this(tycon)
  if defn.isNonRefinedFunction(tp) then
    // Convert toplevel generic function types to dependent functions
    val args0 = args.init
    var res0 = args.last
    val args1 = mapNested(args0)
    val res1 = this(res0)
    // ... do the conversion

It retrieves the argument and the result type of the function directly from the applied type arguments. This works for function classes. However, if the type here is a type alias to the function, e.g. type Lazy[X] = Unit => X, it is still recognized as a function (since defn.isNonRefinedFunction does dealiasing) but it is incorrect to get the argument and result type of the function from the type parameter list here (which is just X :: Nil).

To fix this, we will dealias the type and recurse when it is an alias to the function type.