[lldb][gui] use just '#2' instead of 'frame #2' in the threads/frame view

Since the threads/frame view is taking only a small part on the right side
of the screen, only a part of the function name of each frame is visible.
It seems rather wasteful to spell out 'frame' there when it's obvious
that it is a frame, it's better to use the space for more of the function
name.

Differential Revision: https://reviews.llvm.org/D122998