[lldb] common completion for process pids and process names

1. Added two common completions: `ProcessIDs` and `ProcessNames`, which are
refactored from their original dedicated option completions;
2. Removed the dedicated option completion functions of `process attach` and
`platform process attach`, so that they can use arg-type-bound common
completions instead;
3. Bound `eArgTypePid` to the pid completion, `eArgTypeProcessName` to the
process name completion in `CommandObject.cpp`;
4. Added a related test case.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D80700