[stdlib] Constraining AnySequence.init

... to only allow sequences where SubSequence.SubSequence ==
SubSequence. See swift-evolution proposal
https://github.com/apple/swift-evolution/blob/master/proposals/0014-constrained-AnySequence.md
for more details.

stdlib/public/core/ExistentialCollection.swift.gyb

@@ -224,7 +224,14 @@ public struct AnySequence<Element> : SequenceType {
   public typealias T = Element
 
   /// Wrap and forward operations to `base`.
-  public init<S: SequenceType where S.Generator.Element == Element>(_ base: S) {
+  public init<
+    S: SequenceType
+    where
+      S.Generator.Element == Element,
+      S.SubSequence : SequenceType,
+      S.SubSequence.Generator.Element == Element,
+      S.SubSequence.SubSequence == S.SubSequence
+  >(_ base: S) {
     _box = _SequenceBox(base)
   }