stdlib: make RangeReplaceableCollection.SubSequence a RangeReplaceableCollection

Fixes rdar://problem/28330668.

Author: gribozavr

stdlib/public/core/RangeReplaceableCollection.swift.gyb

@@ -244,6 +244,9 @@ public protocol _RangeReplaceableIndexable : _Indexable {
 public protocol RangeReplaceableCollection
   : _RangeReplaceableIndexable, Collection
 {
+  associatedtype SubSequence : RangeReplaceableIndexable /*: RangeReplaceableCollection*/
+    = RangeReplaceableSlice<Self>
+
   //===--- Fundamental Requirements ---------------------------------------===//
 
   /// Creates a new, empty collection.

validation-test/stdlib/CollectionDiagnostics.swift

@@ -116,3 +116,18 @@ struct BadBidirectionalIndexable : BidirectionalIndexable {
   // expected-error@+1 {{'index(after:)' has different argument names from those required by protocol '_BidirectionalIndexable' ('index(before:)'}}
   func index(after i: Int) -> Int { return 0 }
 }
+
+//
+// Check that RangeReplaceableCollection.SubSequence is defaulted.
+//
+
+struct RangeReplaceableCollection_SubSequence_IsDefaulted : RangeReplaceableCollection {
+  var startIndex: Int { fatalError() }
+  var endIndex: Int { fatalError() }
+
+  func index(after: Int) -> Int { fatalError() }
+  func index(before: Int) -> Int { fatalError() }
+  func index(_: Int, offsetBy: Int) -> Int { fatalError() }
+  func distance(from: Int, to: Int) -> Int { fatalError() }
+}
+