[stdlib] Add availability annotation to other new conformances and members

Author: xwu

stdlib/public/core/DropWhile.swift.gyb

@@ -140,7 +140,9 @@ public struct LazyDropWhileIndex<Base : Collection> : Comparable {
   }
 }
 
+@available(swift, introduced: 4.1) // FIXME(conformance-availability)
 extension LazyDropWhileIndex : Hashable where Base.Index : Hashable {
+  @available(swift, introduced: 4.1)
   public var hashValue: Int {
     return base.hashValue
   }

stdlib/public/core/Flatten.swift

@@ -230,8 +230,10 @@ extension FlattenCollection.Index : Comparable {
   }
 }
 
+@available(swift, introduced: 4.1) // FIXME(conformance-availability)
 extension FlattenCollection.Index : Hashable
   where Base.Index : Hashable, Base.Element.Index : Hashable {
+  @available(swift, introduced: 4.1)
   public var hashValue: Int {
     return _mixInt(_inner?.hashValue ?? 0) ^ _outer.hashValue
   }

stdlib/public/core/PrefixWhile.swift.gyb

@@ -162,7 +162,9 @@ public struct LazyPrefixWhileIndex<Base : Collection> : Comparable {
   }
 }
 
+@available(swift, introduced: 4.1) // FIXME(conformance-availability)
 extension LazyPrefixWhileIndex : Hashable where Base.Index : Hashable {
+  @available(swift, introduced: 4.1)
   public var hashValue: Int {
     switch _value {
     case .index(let value):

stdlib/public/core/Reverse.swift

@@ -193,7 +193,9 @@ extension ReversedCollection.Index: Comparable {
   }
 }
 
+@available(swift, introduced: 4.1) // FIXME(conformance-availability)
 extension ReversedCollection.Index: Hashable where Base.Index: Hashable {
+  @available(swift, introduced: 4.1)
   public var hashValue: Int {
     return base.hashValue
   }