Use Process Plugin register indices when communicating with remote

Summary:
eRegisterKindProcessPlugin is used to store the register
indices used by the remote, and eRegisterKindLLDB is used
to store the internal lldb register indices. However, we're currently
using the lldb indices instead of the process plugin indices
when sending p/P packets. This will break if the remote uses
non-contiguous register indices.

Reviewers: jasonmolenda, clayborg

Subscribers: lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D19305

This is a cherry-pick of r267466

llvm-svn: 270132