Mark tests as unsupported under libcpp-no-exceptions

[rogfer01]

Summary

The destructor of std::promise needs to construct a std::future_error
exception so it calls std::make_exception_ptr. But under
libcpp-no-exceptions this will trigger an abort.

[rmaprath]

LGTM. Needs approval from @EricWF or @mclow.lists.

[rogfer01]

Ping? :)

[EricWF]

I'm OK with this change. Although I'm not 100% sure it's needed, since the XFAIL seems to work for these tests. However using UNSUPPORTED is more consistent with other tests.

test/std/thread/futures/futures.shared_future/dtor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
-// XFAIL: libcpp-no-exceptions
+// UNSUPPORTED: libcpp-no-exceptions
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03
 
 // <future>
 
 // class shared_future<R>
 
 // ~shared_future();

test/std/thread/futures/futures.unique_future/dtor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
-// XFAIL: libcpp-no-exceptions
+// UNSUPPORTED: libcpp-no-exceptions
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03
 
 // <future>
 
 // class future<R>
 
 // ~future();