[lldb] Move SwiftLanguageRuntime to its own plugin

[bulbazord]

Motivation: I want to move swift-specific behavior into plugins in order to reduce friction between upstream lldb and the swift variant. Moving swift-specific classes out of Target into plugins is a good place to start I think.

cc @kastiglione @adrian-prantl

[adrian-prantl]

Thanks, I think this is a long-overdue refactoring, but we need to do this on all branches (i.e., also on the swift-5.5 branch) at the same time or we'll end up with a lot of merge conflicts. There is never a good time to make these kinds of changes, but by doing them on all branches at the same time, we can minimize the impact.

[kastiglione]

also swift/next

[bulbazord]

Once this change looks good to y'all, I can create PRs for other branches in order to try to reduce the amount of friction. Just let me know if there's anything else that can be improved/changed.

[JDevlieghere]

This LGTM

[bulbazord]

I opened PRs for swift/next and swift/release/5.5. Let me know if there are other branches.

swift/next: #2899
swift/release/5.5: #2898 and #2887

[adrian-prantl]

Everything else should be handled by the automerger. Thanks!

[JDevlieghere]

@swift-ci please test

edit: ugh, didn't check the target branch.

[bulbazord]

@JDevlieghere Could you also merge this one when you have the time? Thanks!

[JDevlieghere]

The automerger from 5.5 -> main should take care of that for us.

[bulbazord]

Awesome, I'll close this PR then.