[lldb] Tab completion for process plugin name

Summary:

1. Added tab completion to `process launch -p`, `process attach -P`, `process
connect -p`;

2. Bound the plugin name common completion as the default completion for
`eArgTypePlugin` arguments.

Reviewers: teemperor, JDevlieghere

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D79929