Clang supports coroutines in all dialects; Therefore libc++ should too,
otherwise the Clang extension is unusable.
I'm not convinced extending support to C++03 is a feasible long term
plan, since as the library grows to offer things like generators it
will be come increasingly difficult to limit the implementation to C++03.
However for the time being supporting C++03 isn't a big deal.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@303963 91177308-0d34-0410-b5e6-96231b3b80d8
4aec787d15