Implement MachOObjectFile::getHeaderSize and MachOObjectFile::getData.

These were the last missing forwarding functions. Also consistently use
the forwarding functions instead of using MachOObj directly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178992 91177308-0d34-0410-b5e6-96231b3b80d8