Maintain register numbering across xml include features

Summary:
If the remote uses include features when communicating
xml register info back to lldb, the existing code would reset the
lldb register index at the beginning of each include node.
This would lead to multiple registers having the same lldb register index.
Since the lldb register numbers should be contiguous and unique,
maintain them accross the parsing of all of the xml feature nodes.

Reviewers: jingham, jasonmolenda, clayborg

Subscribers: lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D19303

This is a cherry-pick of 267468

llvm-svn: 270134