[mach-o] Fix MachOFileNode to own archives same as ELFFileNode

My previous fix to have FileArchive own the member MemoryBuffers was not a
complete solution for darwin because nothing owned the FileArchive object.
Fixed MachOFileNode to be like ELFFileNode and have the graph node own the
archive object.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221552 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] remove stray debug output

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221553 91177308-0d34-0410-b5e6-96231b3b80d8