[coroutines] Add std::experimental::task<T> type

Summary:
Adds the coroutine `std::experimental::task<T>` type described in proposal P1056R0.
See https://wg21.link/P1056R0.

This implementation allows customization of the allocator used to allocate the
coroutine frame by passing std::allocator_arg as the first argument, followed by
the allocator to use.

This supports co_awaiting the same task multiple times. The second and
subsequent times it returns a reference to the already-computed value.

This diff also adds some implementations of other utilities that have potential for
standardization as helpers within the test/... area:
- `sync_wait(awaitable)` - See P1171R0
- `manual_reset_event`

Move the definition of the __aligned_allocation_size helper function
from <experimental/memory_resource> to <experimental/__memory>
so it can be more widely used without pulling in memory_resource.

Outstanding work:
- Use C++14 keywords directly rather than macro versions
  eg. use `noexcept` instead of `_NOEXCEPT`).
- Add support for overaligned coroutine frames.
  This may need wording in the Coroutines TS to support passing the extra `std::align_val_t`.
- Eliminate use of `if constexpr` if we want it to compile under C++14.

Patch by @lewissbaker (Lewis Baker).

llvm-svn: 357010

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