Foundation overlay: Don't rely on URL struct being a single reference

If the struct is resilient, it is passed indirectly. Even if we decide to
make it fixed layout, it's best to remove as many implicit ABI assumptions
as possible from @_silgen_name function calls.

Fixes <rdar://problem/28673618>.

Author: slavapestov

stdlib/public/SDK/Foundation/Data.swift

@@ -22,7 +22,7 @@ internal func __NSDataInvokeDeallocatorUnmap(_ mem: UnsafeMutableRawPointer, _ l
 internal func __NSDataInvokeDeallocatorFree(_ mem: UnsafeMutableRawPointer, _ length: Int) -> Void
 
 @_silgen_name("_NSWriteDataToFile_Swift")
-internal func _NSWriteDataToFile_Swift(url: URL, data: NSData, options: UInt, error: NSErrorPointer) -> Bool
+internal func _NSWriteDataToFile_Swift(url: NSURL, data: NSData, options: UInt, error: NSErrorPointer) -> Bool
 
 internal final class _SwiftNSData : _SwiftNativeNSData, _SwiftNativeFoundationType {
     internal typealias ImmutableType = NSData
@@ -385,7 +385,7 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
         try _mapUnmanaged {
             if _shouldUseNonAtomicWriteReimplementation(options: options) {
                 var error : NSError?
-                if !_NSWriteDataToFile_Swift(url: url, data: $0, options: options.rawValue, error: &error) {
+                if !_NSWriteDataToFile_Swift(url: url as NSURL, data: $0, options: options.rawValue, error: &error) {
                     throw error!
                 }
             } else {