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

This revision is causing build and test failures, such as
http://lab.llvm.org:8011/builders/libcxx-libcxxabi-libunwind-armv8-linux/builds/648/steps/test.libcxx/logs/stdio,
so I'll revert it.

llvm-svn: 357023