[Binary] Promote OffloadBinary to inherit from Binary

We use the `OffloadBinary` to create binary images of offloading files
and their corresonding metadata. This patch changes this to inherit from
the base `Binary` class. This allows us to create and insepect these
more generically. This patch includes all the necessary glue to
implement this as a new binary format, along with added the magic bytes
we use to distinguish the offloading binary to the `file_magic`
implementation.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D126812