Add MachOObjectFile::getUuid()

This CL introduces MachOObjectFile::getUuid(). This function returns an ArrayRef to the object file's UUID, or an empty ArrayRef if the object file doesn't contain an LC_UUID load command.
The new function is gonna be used by llvm-symbolizer.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219866 91177308-0d34-0410-b5e6-96231b3b80d8