Refactor UnwindLLDB so it doesn't populate the entire stack unless

the frame count is requested or each frame is individually requested.

In practice this doesn't seem to help anything because we have
functions like StackFrameList::GetNumFrames() which is going to
request each frame anyway.  And classes like ThreadPlanStepRange
and ThreadPlanStepOverRange get the stack depth in their ctor forcing
a full stack walk.  But at least UnwindLLDB will delay doing a full
walk if it can.

llvm-svn: 118477