Fixed a kernel panic that would occur if you debug anything on MacOSX 10.7 or...

Fixed a kernel panic that would occur if you debug anything on MacOSX 10.7 or earlier due to the use of the POSIX_SPAWN_CLOEXEC_DEFAULT attribute flag that closes all file descriptors on exec. We now dyamically detect the OS version and do the right thing.

llvm-svn: 198776