Fix bug in ArchSpec::MergeFrom

Previous ArchSpec tests didn't catch this bug since we never tested just the OS being out of date. Fixed the bug and covered this with a test that would catch this.

This was found when trying to load a core file where the core file was an ELF file with just the e_machine for architeture and where the ELF header had no OS set in the OSABI field of the e_ident. It wasn't merging the architecture with the target architecture correctly.

Differential Revision: https://reviews.llvm.org/D61659

llvm-svn: 360292