ClearCase: Give warning if editing a Derived Object

Derived objects (DO) are created during builds with ClearCase build
tools (e.g., clearmake). They are used for build avoidance and derived
object sharing. DOs are typically object files, but can also be source
code generated from code generation tools. It is probably never a good
idea to edit such a file.

Change-Id: If4248f09409424ee771aab720851bb6ce1aa40c9
Reviewed-by: Orgad Shaneh <orgads@gmail.com>