Remove static initializer from DataStream.cpp

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150120 91177308-0d34-0410-b5e6-96231b3b80d8