[lldb] Remove distribution_id from ArchSpec

The qHostInfo packet in the gdb-remote communication protocol specifies
that distribution_id can be set, so lldb handles that. But we store that
in the ArchSpec representing the "Host" platform (whatever platform the
debug server is running on). This field is otherwise unused in ArchSpec,
so it would be a lot easier if we stored that information at the
gdb-remote communication layer.

Sidenote: The distribution_id field is currently unused but I did not
want to remove it in case some folks found it useful (e.g. in downstream
forks).

Differential Revision: https://reviews.llvm.org/D149697