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.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357023 91177308-0d34-0410-b5e6-96231b3b80d8
15 files changed