Fix thread/process ID reading from linux core files

Summary:
This also adds a basic smoke test for linux core file reading. I'm checking in the core files as
well, so that the tests can run on all platforms. With some tricks I was able to produce
reasonably-sized core files (~40K).

This fixes the first part of pr26322.

Reviewers: zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D18176

llvm-svn: 263628