[asan] Set abort_on_error=1 by default on OS X

This sets the default ASan flags to abort_on_error=1 on OS X. For unit tests and lit tests we set ASAN_OPTIONS back to abort_on_error=0 before running the tests (to avoid crashing). I added two tests that intentionally don't respect the default ASAN_OPTIONS to test the behavior of an empty ASAN_OPTIONS (on OS X we should crash, on Linux we should exit()).

Differential Revision: http://reviews.llvm.org/D7203

llvm-svn: 243418