swift234

Sema: Fix type safety hole with inherited conformances

When checking for permitted uses of Self in the input type of a
protocol requirement's function type, if the parameter itself was
a function we would recurse into its input, and reject all uses
of Self in the parameter type's result. This was the wrong way
around, and in fact we should recurse into the result.

Here is a test case that used to compile successfully and crash;
now it is rejected by the type checker:

    protocol P {
      func f(a: Self -> ())
    }

    protocol Q : P {
      func g()
    }

    class C : P {
      func f(a: C -> ()) { // should not be allowed to witness P.f
        a(C())
      }
    }

    class B : C, Q {
      var x: Int = 17

      func g() {
        print(x)
      }
    }

    func f<T : Q>(t: T) {
      // T == B here
      // t.f has type <T : Q> (T -> ()) -> ()
      t.f({ $0.g() }) // but at runtime, $0 is a C not a B
    }

    f(B())