[lldb] watchpoint ID common completion for commands `watchpoint...

[lldb] watchpoint ID common completion for commands `watchpoint delete/enable/disable/modify/ignore`

1. Added a common completion WatchPointIDs to complete with a list of the IDs of the current watchpoints;
2. Applied the completion to these commands: watchpoint delete/enable/disable/modify/ignore;
3. Added a correlated test case.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D84104