Build a simple native MachO reader.

[`ELFObjectFile`](https://github.com/llvm/llvm-project/blob/e3cbd9984a78422c3799629eb6b5f7f7818c1a11/llvm/include/llvm/Object/ELFObjectFile.h#L268) has been refactored to become a wrapper around [`ELFFile`](https://github.com/llvm/llvm-project/blob/e3cbd9984a78422c3799629eb6b5f7f7818c1a11/llvm/include/llvm/Object/ELF.h#L257), which provides a simple native reader for the ELF format.

MachO should adopt a similar scheme, since a native reader can be much faster. This would be helpful for performance-sensitive MachO clients (e.g. the LLVM JIT).