Remove object_error::success and use std::error_code() instead

make_error_code(object_error) is slow because object::object_category()
uses a ManagedStatic variable. But the real problem is that the function is
called too frequently. This patch uses std::error_code() instead of
object_error::success. In most cases, we return "success", so this patch
reduces number of function calls to that function.

http://reviews.llvm.org/D10333

llvm-svn: 239409