Use error_code() instead of error_code::succes()

There is no std::error_code::success, so this removes much of the noise
in transitioning to std::error_code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209949 91177308-0d34-0410-b5e6-96231b3b80d8