[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).



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@175059 91177308-0d34-0410-b5e6-96231b3b80d8