skip copying old values during removeAll()

As noted in FIXME(performance), calling `.removeAll(keepCapacity: true)` on a
containter of type Dictionay or Set causes a copy of its storage being made.
Only then, it would proceed to delete each element.

This patch makes these hashed collections skip the wasteful copying step.

Author: dduan

stdlib/public/core/HashedCollections.swift.gyb

@@ -3393,27 +3393,21 @@ internal enum _Variant${Self}Storage<${TypeParametersDecl}> : _HashStorageType {
     }
   }
 
-  internal mutating func nativeRemoveAll() {
-    var nativeStorage = native
-
-    // FIXME(performance): if the storage is non-uniquely referenced, we
-    // shouldn’t be copying the elements into new storage and then immediately
-    // deleting the elements. We should detect that the storage is not uniquely
-    // referenced and allocate new empty storage of appropriate capacity.
+  internal mutating func nativeKeepCapacityRemoveAll() {
 
     // We have already checked for the empty dictionary case, so we will always
     // mutating the dictionary storage.  Request unique storage.
-    let (reallocated, _) = ensureUniqueNativeStorage(nativeStorage.capacity)
-    if reallocated {
-      nativeStorage = native
-    }
 
-    for var b = 0; b != nativeStorage.capacity; ++b {
-      if nativeStorage.isInitializedEntry(b) {
-        nativeStorage.destroyEntryAt(b)
+    if !isUniquelyReferenced() {
+      self = .Native(NativeStorageOwner(minimumCapacity: native.capacity))
+    } else {
+      for b in 0..<native.capacity {
+        if native.isInitializedEntry(b) {
+          native.destroyEntryAt(b)
+        }
       }
     }
-    nativeStorage.count = 0
+    native.count = 0
   }
 
   internal mutating func removeAll(keepCapacity keepCapacity: Bool) {
@@ -3427,13 +3421,13 @@ internal enum _Variant${Self}Storage<${TypeParametersDecl}> : _HashStorageType {
     }
 
     if _fastPath(guaranteedNative) {
-      nativeRemoveAll()
+      nativeKeepCapacityRemoveAll()
       return
     }
 
     switch self {
     case .Native:
-      nativeRemoveAll()
+      nativeKeepCapacityRemoveAll()
     case .Cocoa(let cocoaStorage):
 #if _runtime(_ObjC)
       self = .Native(NativeStorage.Owner(minimumCapacity: cocoaStorage.count))