[stdlib] Restore Array value semantics

'nuff said.

Swift SVN r18923

stdlib/core/Arrays.swift.gyb

@@ -42,10 +42,25 @@ struct ${Self}<T> : MutableCollection, Sliceable {
       _precondition(index >= 0, "Negative ${Self} index is out of range")
       return _buffer[index]
     }
-    nonmutating set {
+    set {
       _precondition(index < count, "${Self} index out of range")
       _precondition(index >= 0, "Negative array index is out of range")
-      _buffer[index] = newValue
+      if _buffer.isMutableAndUniquelyReferenced() {
+        _buffer[index] = newValue
+      }
+      else {
+        {
+          (inout buffer: _Buffer, e: Element)->() in
+          var newBuffer = ContiguousArrayBuffer<T>(
+            count: buffer.count, minimumCapacity: buffer.count)
+          let target = buffer._uninitializedCopy(
+            0..index, target: newBuffer.elementStorage)
+          target.initialize(e)
+          buffer._uninitializedCopy(
+            (index + 1)..buffer.count, target: target + 1)
+          buffer = _Buffer(newBuffer)
+        }(&_buffer, newValue)
+      }
     }
   }
 
@@ -147,15 +162,12 @@ extension ${Self} : ArrayType {
   /// array.
   /// Complexity: O(N)
   func copy() -> ${Self} {
-    var result = self
-    result.reserveCapacity(0)
-    return result
+    return self
   }
 
   /// Ensure the uniqueness of the array.
   /// Complexity: O(N)
-  mutating func unshare() {
-    reserveCapacity(0)
+  func unshare() {
   }
 
   /// Ensure the array has enough mutable contiguous storage to store