Add fixed crasher from SR-5905.

Author: DougGregor

validation-test/compiler_crashers_2_fixed/0126-sr5905.swift

@@ -0,0 +1,28 @@
+// RUN: not %target-swift-frontend %s -typecheck
+protocol VectorIndex {
+    associatedtype Vector8 : Vector where Vector8.Index == Self, Vector8.Element == UInt8
+}
+enum VectorIndex1 : VectorIndex {
+    case i0
+    typealias Vector8 = Vector1<UInt8>
+}
+protocol Vector {
+    associatedtype Index: VectorIndex
+    associatedtype Element
+    init(elementForIndex: (Index) -> Element)
+    subscript(index: Index) -> Element { get set }
+}
+struct Vector1<Element> : Vector {
+    //typealias Index = VectorIndex1 // Uncomment this line to workaround bug.
+    var e0: Element
+    init(elementForIndex: (VectorIndex1) -> Element) {
+        e0 = elementForIndex(.i0)
+    }
+    subscript(index: Index) -> Element {
+        get { return e0 }
+        set { e0 = newValue }
+    }
+}
+extension Vector where Index == VectorIndex1 {
+    init(_ e0: Element) { fatalError() }
+}