[ASan] When re-executing the process on OS X, make sure we update the existing...

[ASan] When re-executing the process on OS X, make sure we update the existing DYLD_INSERT_LIBRARIES correctly.
Previously ASan used to hang in an exec loop, because it failed to overwrite the env var value
(see https://code.google.com/p/address-sanitizer/issues/detail?id=159).

llvm-svn: 175059