Reapply "Use gethostuuid() on Mac to identify hosts for LockFileManager"

Reapplies r241005 after fixing the build on non-Mac platforms. Original
commit message below.

The hostname can be very unstable when there are many machines on the
network competing for the same name. Using the hardware UUID makes it
less likely to have collisions or to consider files written by the
current host to be owned by a different one at a later time.

rdar://problem/21512307

llvm-svn: 241012