[lldb][nfc] Change ProcessGDBRemote::ParseMultiMemReadPacket signature (#172020)
Instead of returning an `Expected<vector<...>>` it now returns an Error,
and receives a vector argument to fill in. This will be useful to
support a change were ParseMultiMemReadPacket will be called multiple
times in a loop with the same vector; without this change, we would have
to concatenate vectors and copy memory around.