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

lldb/source/Target/StackFrameList.cpp

@@ -802,6 +802,7 @@ StackFrameList::GetStackFrameSPForStackFramePtr(StackFrame *stack_frame_ptr) {
 size_t StackFrameList::GetStatus(Stream &strm, uint32_t first_frame,
                                  uint32_t num_frames, bool show_frame_info,
                                  uint32_t num_frames_with_source,
+                                 bool show_unique,
                                  const char *selected_frame_marker) {
   size_t num_frames_displayed = 0;
 
@@ -842,7 +843,7 @@ size_t StackFrameList::GetStatus(Stream &strm, uint32_t first_frame,
 
     if (!frame_sp->GetStatus(strm, show_frame_info,
                              num_frames_with_source > (first_frame - frame_idx),
-                             marker))
+                             show_unique, marker))
       break;
     ++num_frames_displayed;
   }