I'm working on packaging Swift for Fedora (https://bugzilla.redhat.com/show_bug.cgi?id=1536780) and the question has come up about whether Swift's lldb can support Python 3. The llvm.org's page for lldb indicates that they use Python 3; curious if there are any thoughts to switch Swift's version to it as well.
The Swift LLDB should be a superset of the llvm.org LLDB, so (bugs notwithstanding) what works with the llvm.org LLDB should also work with the Swift LLDB. Note that we may not do any testing with Python 3, so be sure to let us know of any issues you run into.
As a general not for packaging: The Swift LLDB should be a drop-in replacement for the normal LLDB since it can do everything regular LLDB can do + Swift.
On rolling distros or other distributions with relatively recent packages that might be problematic, as the LLDB version shipped with Swift might be months behind the current upstream release.
This is of course something you'll need to re-evaluate for every release, but in the recent past we've branched LLDB at the same point as the llvm.org stable releases were.