[lld-macho] Support parsing of bitcode within archives

Also error out if we find anything other than an object or bitcode file
in the archive.

Note that we were previously inserting the symbols and sections of the
unpacked ObjFile into the containing ArchiveFile. This was actually
unnecessary -- we can just insert the ObjectFile (or BitcodeFile) into
the `inputFiles` vector. This is the approach taken by LLD-ELF.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D92539