Need help with llvm-toolchain-3.8 on armel

[Sylvestre Ledru]

Hello,

Is there anyone to help with this bug 
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=820535 ?
LLVM toolchain 3.8 and later (3.9 & 4.0) are failing with:

| /«PKGBUILDDIR»/include/llvm/Support/ThreadPool.h: In member function 
'std::shared_future<void> llvm::ThreadPool::async(Function&&, Args&& ...)':
| /«PKGBUILDDIR»/include/llvm/Support/ThreadPool.h:77:77: error: return type 
'class std::shared_future<void>' is incomplete
|    inline std::shared_future<VoidTy> async(Function &&F, Args &&... ArgList) {

I don't know what is wrong here and this is blocking the llvm migration.

Thanks,
Sylvestre