[stdlib] Make stdlib index types Hashable

This adds Hashable conformance for the String, Dictionary,
and Set's index types.

Author: natecook1000

stdlib/public/core/HashedCollections.swift.gyb

@@ -5967,7 +5967,7 @@ elif Self == 'Dictionary':
 
 ${SubscriptingWithIndexDoc}
 @_fixed_layout // FIXME(sil-serialize-all)
-public struct Index : Comparable {
+public struct Index : Comparable, Hashable {
   // Index for native buffer is efficient.  Index for bridged NS${Self} is
   // not, because neither NSEnumerator nor fast enumeration support moving
   // backwards.  Even if they did, there is another issue: NSEnumerator does
@@ -6089,6 +6089,22 @@ extension ${Self}.Index {
   #endif
     }
   }
+
+  @_inlineable // FIXME(sil-serialize-all)
+  public var hashValue: Int {
+    if _fastPath(_guaranteedNative) {
+      return _nativeIndex.offset
+    }
+
+    switch _value {
+    case ._native(let nativeIndex):
+      return nativeIndex.offset
+  #if _runtime(_ObjC)
+    case ._cocoa(let cocoaIndex):
+      return cocoaIndex.currentKeyIndex
+  #endif
+    }
+  }
 }
 
 #if _runtime(_ObjC)

stdlib/public/core/StringIndex.swift

@@ -67,6 +67,13 @@ extension String.Index : Comparable {
   }
 }
 
+extension String.Index : Hashable {
+  @_inlineable // FIXME(sil-serialize-all)
+  public var hashValue: Int {
+    return _compoundOffset.hashValue
+  }
+}
+
 extension String.Index {
   internal typealias _Self = String.Index