[ObjectYAML] Pull DWARF support into DWARFYAML namespace

Since DWARF formatting is agnostic to the object file it is stored in, it doesn't make sense for this to be in the MachOYAML implementation. Pulling it into its own namespace means we could modify the ELF and COFF YAML tools to emit DWARF as well.

In a follow-up patch I will better abstract this in obj2yaml and yaml2obj so that the DWARF bits in the tools can be re-used too.

llvm-svn: 288984