[PECOFF] Remove special treatment of "--" option.

Thanks to Hans' patch (r187675), OptTable now handles "--", so we don't
need this code in LLD.
ObjectiveC migrator. Differentiate 'instancetype'
from 'id' result type when deciding on migration
to instancetype.
Add a AttributeSetImpl::dump function.

This is for the benefit of those of us with inferior debuggers which
do not permit member function calls on value types.