[stdlib] On-demand unique contiguous Array storage

Lift the precondition that contiguous storage already exist on the use
of Array.withUnsafe[Mutable]PointerToElements.  Automatically guarantee
that storage is unique when using the mutable variants of these methods.

Also, fix some---essentially---inout aliasing violations in
test/stdlib/Unicode.swift that were revealed by the new careful
implementation of these methods.

Swift SVN r20339

stdlib/core/ArrayBuffer.swift

@@ -354,26 +354,30 @@ extension _ArrayBuffer {
     }
   }
 
-  /// Call body(p), where p is a pointer to the underlying contiguous storage
-  /// Requires: such contiguous storage exists or the buffer is empty
+  /// Call `body(p)`, where `p` is a pointer to the underlying
+  /// contiguous storage.  If no contiguous storage exists, it is
+  /// created on-demand.
   public
   func withUnsafePointerToElements<R>(
     body: (UnsafePointer<T>)->R
   ) -> R {
-    _precondition(
-      elementStorage != nil || count == 0,
-      "Array is bridging an opaque NSArray; can't get a pointer to the elements"
-    )
-    let ret = body(elementStorage)
+    if _isClassOrObjCExistential(T.self) {
+      if _nonNative {
+        indirect.replaceStorage(_copyCollectionToNativeArrayBuffer(self))
+      }
+    }
+    let ret = body(self.elementStorage)
     _fixLifetime(self)
     return ret
   }
   
+  /// Call `body(p)`, where `p` is a pointer to the underlying contiguous
+  /// storage.  Requires: Contiguous storage already exists
   public
   mutating func withUnsafeMutablePointerToElements<R>(
     body: (UnsafeMutablePointer<T>)->R
   ) -> R {
-    _precondition(
+    _sanityCheck(
       elementStorage != nil || count == 0,
       "Array is bridging an opaque NSArray; can't get a pointer to the elements"
     )