Introduce new command: thread backtrace unique

This patch introduces a new thread backtrace command "unique".
The command is based off of "thread backtrace all" but will instead
find all threads which share matching call stacks and de-duplicate
their output, listing call stack and all the threads which share it.
This is especially useful for apps which use thread/task pools
sitting around waiting for work and cause excessive duplicate output.
I needed this behavior recently when debugging a core with 700+ threads.

Differential Revision: https://reviews.llvm.org/D33426

Reviewers: clayborg, jingham
Patch by Brian Gianforcaro <b.gianfo@gmail.com>

llvm-svn: 305197