[lldb] thread index common completion for commands like `thread select/step-over`

1. Added a common completion completing with a list of the threads of the current process;
2. Apply the common completion above to these commands: thread
   continue/info/exception/select/step-in/step-inst/step-inst-over/step-out/step-over/step-script​
3. Correlated test case test_common_completion_thread_index.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D84088