First chunk of native object file reader/writer. The lld-core tool now reads...

First chunk of native object file reader/writer.  The lld-core tool now reads YAML file, links, writes that out as native object format, then reads that native file, then writes the YAML to stdout.  Thus the test suite tests both YAML reading/writing as well as native object file reading/writing.

llvm-svn: 148256