Remove static initializer from DataStream.cpp

If someone would prefer a clear name for the 'success' error_value we could
come up with one - potentially just a 'named constructor' style
'error_value::success()' to make this expression more self-documenting. If
I see this come up in other cases I'll certainly consider it.

One step along the way to resolving PR11944.

llvm-svn: 150120