Update coroutine_handle<P>::promise to reflect N4663.

This patch updates the promise() member to match the current spec.
Specifically it removes the non-const overload and make the return
type of the const overload non-const.

This patch also makes the ASSERT_NOT_NOEXCEPT tests libc++ specific,
since other implementations may be free to strengthen the specification.

llvm-svn: 303895