Fix build errors on some configurations

It's been reported
<http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20180611/559616.html>
that template argument deduction for RetryAfterSignal fails if open is
not prefixed with "::".

This should help us build correctly on those platforms and explicitly
specifying the namespace is more correct anyway.

llvm-svn: 334403