[PATCH] [dfsan] Introduce dfsan_union runtime functionn.

[Peter Collingbourne]

  I think I brought this up somewhere else before but:

  - dfsan_ functions are intended to be called by the user, while __dfsan_ functions are called implicitly by the compiler.  The naming convention is documented at the top of dfsan.cc and makes it clear which is which.
  - I think the dfsan_ prefix is enough to avoid namespace pollution.

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