Fix type specialization during code generation

[kyouko-taiga]

The following program crashes the compiler with an assertion failure (b5bd439):

trait P {
  type T: SemiRegular
  type U

  fun t(_ x: T)
  fun u(_ x: U)
}

trait Q: SemiRegular {
  type V: P
  property v: V { let }
}

type A<V: P & SemiRegular>: Q {
  public let _v: V
  public init (v: sink V) { &self._v = v }
  public property v: V { yield _v }
  public fun infix== (_ other: Self) -> Bool { self.v == other.v }
}

type B<W: Q>: Deinitializable {
  public let w: W
  public let tt: W.V.T
  public memberwise init

  public fun t(_ x: W.V.T) { w.v.t(x) }
  public fun u(_ x: W.V.U) { w.v.u(x) }
}

conformance Array: P {
  public typealias T = Element
  public typealias U = Self.Position

  public fun t(_ x: Element) {}
  public fun u(_ x: Self.Position) {}
}

public fun main() {
  let a = A(v: Array<Bool>())
  let b = B(w: a, tt: true)
  b.t(true)
  b.u(1)
}

Note: this program is adapted from this test.

The failure occurs in AbstractTypeLayout.swift and it is due to the layout B<A<Array<Bool>>> containing an error type. This error type is produced by the specialization of the generic type, which occurs in TypedProgram.storage(of:), under the "FIXME" note.