Prevent client from querying each thread's PC at each stop.

Summary:
The server was no longer sending the thread PCs the way the client
expected them.
I changed the server to send them back as a threadstop info field,
similar to the Apple version of the server.
I also changed the client to look for them there, before querying the
server.
I added a test to ensure the server doesn't stop sending them.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D28880
Author: Jason Majors

llvm-svn: 292611