Issue 3882: Add warning for unused returns of strictly pure function calls

This works for me in my projects and actually detected two bugs for me.

I'm new to Git and I have accidentally added support for Exuberant Ctags tag generation. If you don't like it tell me and I'll remove and do a new pull request.

An acompanying pull request for Phobos to prevent a handful of warnings generated by this change is pending aswell.