gc: produce collection report on success

[jorgeorpinel]

When I run dvc gc or/and dvc gc --cloud after removing some data from DVC control, it outputs and prompts:

WARNING: this will remove all cache except the cache that is used in the current git branch of the current repo.
Are you sure you want to proceed? [y/n] 

Then, when I say y it just completes the operation and exits with no more output. Maybe a report of what was collected (and from where) would be more informative. This way the user could know something actually happened, without having to understand and check DVC internals before and after running the command.

Per https://discordapp.com/channels/485586884165107732/563406153334128681/616421757808541721

[jorgeorpinel]

Maybe something to include in #2325

[efiop]

Closing in favor of #2325