Reenable temporarily disabled test.

The test relies on new runtime functionality that is by definition not
available in the stdlib in the OS.  Here, the test is marked as
unsupported run running using the OS' stdlib.

rdar://problem/59425215

Author: nate-chandler

test/Prototypes/Result_future.swift

@@ -12,7 +12,6 @@
 // RUN: %target-run-stdlib-swift-target-future
 // REQUIRES: executable_test
 
-// REQUIRES: rdar59425215
 // REQUIRES: OS=macosx || OS=ios || OS=tvos || OS=watchos || OS=linux-gnu
 // UNSUPPORTED: CPU=i386 && OS=ios
 // UNSUPPORTED: CPU=armv7 && OS=ios
@@ -21,6 +20,7 @@
 // UNSUPPORTED: CPU=x86_64 && OS=ios
 // UNSUPPORTED: CPU=x86_64 && OS=tvos
 // UNSUPPORTED: CPU=i386 && OS=watchos
+// UNSUPPORTED: use_os_stdlib
 
 public enum Result<Value> {
 case Success(Value)