[mach-o] add representation for LC_ID_DYLIB to MachONormalizedFile

It still needs to be tied into BinaryReader, but this allows reasonably
sensible creation of SharedLibrary atoms on MachO.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@212093 91177308-0d34-0410-b5e6-96231b3b80d8