[libcxx] Mark most test/std/future tests as UNSUPPORTED in C++03

Summary:
This patch marks *most* tests for `std::promise`, `std::future` and `std::shared_future` as unsupported in C++03. These tests fail in C++03 mode because they attempt to copy a `std::future` even though it is a `MoveOnly` type. AFAIK the missing move-semantics in `std::future` is the only reason these tests fail but without move semantics these classes are useless. For example even though `std::promise::set_value` and `std::promise::set_exception(...)` work in C++03 `std::promise` is still useless because we cannot call `std::promise::get_future(...)`.

It might be possible to hack `std::move(...)` like we do for `std::unique_ptr` to make the move semantics work but I don't think it is worth the effort. Instead I think we should leave the `<future>` header as-is and mark the failing tests as `UNSUPPORTED`. I don't believe there are any users of `std::future` or `std::promise` in C++03 because they are so unusable. Therefore I am not concerned about losing test coverage and possibly breaking users. However because there are still parts of `<future>` that work in C++03 it would be wrong to `#ifdef` out the entire header.

@mclow.lists Should we take further steps to prevent the use of `std::promise`, `std::future` and `std::shared_future` in C++03?


Note: This patch also cleans up the tests and converts them to use `support/test_allocator.h` instead of a duplicate class in `test/std/futures/test_allocator.h`.

Reviewers: mclow.lists

Subscribers: vsk, mclow.lists, cfe-commits

Differential Revision: http://reviews.llvm.org/D12135

llvm-svn: 246271

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