[PATCH] ignore --disable-free in clang when leak detection is enabled

[Kostya Serebryany]

  >> Isn't clang multithreaded? What about synchronization?
  no (afaict)
  If it ever becomes multithreaded, we can guard the push_backs by a mutex (will add a tiny bit of ugliness)

http://llvm-reviews.chandlerc.com/D2475