[PATCH] [dfsan] Introduce dfsan_union runtime functionn.

[Evgeniy Stepanov]

  LGTM.

  Other sanitizers prefix all user-visible symbols with __ (like __asan_do_something). I think we do this to avoid polluting the global namespace, being part of the compiler runtime library. WDYT?

http://llvm-reviews.chandlerc.com/D1347