[Object] Fix MachO's getUuid to return a pointer into the object instead of a dangling ArrayRef.

This works because uuid's are always little endian so it's not swapped.
Fixes use-after-return reported by asan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220567 91177308-0d34-0410-b5e6-96231b3b80d8