Complete breakpoint enable/disable/delete/modify with a list of breakpoint IDs

Summary:
1. A new common completion `CommandCompletions::Breakpoints` to provide a list of the breakpoints of the current context;
2. Apply the completion above to the commands breakpoint enable/disable/delete/modify;
3. Unit test.

Reviewers: teemperor, JDevlieghere

Reviewed By: teemperor

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D79666