Add a test for a SILGen bug that got fixed with recent generics changes

Author: slavapestov

test/SILGen/generic_witness.swift

@@ -22,3 +22,34 @@ func bar(_ x: Runcible) {
   // CHECK: apply [[METHOD]]<[[OPENED]], Int>
   x.runce(5)
 }
+
+protocol Color {}
+
+protocol Ink {
+    associatedtype Paint
+}
+
+protocol Pen {}
+
+protocol Pencil : Pen {
+    associatedtype Stroke : Pen
+}
+
+protocol Medium {
+    associatedtype Texture : Ink
+
+    func draw<P : Pencil>(paint: Texture.Paint, pencil: P) where P.Stroke == Texture.Paint
+}
+
+struct Canvas<I : Ink> where I.Paint : Pen {
+    typealias Texture = I
+
+    func draw<P : Pencil>(paint: I.Paint, pencil: P) where P.Stroke == Texture.Paint { }
+}
+
+extension Canvas : Medium {}
+
+// CHECK-LABEL: sil hidden [transparent] [thunk] @_TTWuRx15generic_witness3Inkwx5PaintS_3PenrGVS_6Canvasx_S_6MediumS_FS4_4drawuRd__S_6Pencilwd__6StrokezWx7TextureS1__rfT5paintWxS7_S1__6pencilqd___T_ : $@convention(witness_method) <I where I : Ink, I.Paint : Pen><P where P : Pencil> (@in I.Paint, @in P, @in_guaranteed Canvas<I>) -> () {
+// CHECK: [[FN:%.*]] = function_ref @_TFV15generic_witness6Canvas4drawuRd__S_6Pencilwx5Paintzwd__6StrokerfT5paintwxS2_6pencilqd___T_ : $@convention(method) <τ_0_0 where τ_0_0 : Ink, τ_0_0.Paint : Pen><τ_1_0 where τ_1_0 : Pencil, τ_0_0.Paint == τ_1_0.Stroke> (@in τ_0_0.Paint, @in τ_1_0, Canvas<τ_0_0>) -> ()
+// CHECK: apply [[FN]]<I, P, I.Paint>({{.*}}) : $@convention(method) <τ_0_0 where τ_0_0 : Ink, τ_0_0.Paint : Pen><τ_1_0 where τ_1_0 : Pencil, τ_0_0.Paint == τ_1_0.Stroke> (@in τ_0_0.Paint, @in τ_1_0, Canvas<τ_0_0>) -> ()
+// CHECK: }