I looked at this today and I understand what's wrong. I wasn't really familiar with this whole machinery, so it was fun learning about it :)
During the migration from Xcode to CMake the install target has been implemented using "cache" files https://github.com/apple/swift/pull/25931
The cache file for macOS hardcodes the LLDB.framework install path to be
# Choose the install location for LLDB.framework so that it matches the
# INSTALL_RPATH of the lldb driver. It's either absolute or relative to
# CMAKE_INSTALL_PREFIX. In any case, DESTDIR will be an extra prefix.
set(LLDB_FRAMEWORK_INSTALL_DIR /Applications/Xcode.app/Contents/SharedFrameworks CACHE STRING "")
Which is obviously wrong in the case of a nightly toolchain. It should be something like
So, LLDB.framework is not copied in the right place, which makes debugging not functional :)
I have a WIP patch, I need to do some additional testing to make sure this doesn't break internal use-cases (e.g. Xcode) & Linux. I'll try to have the patch merged tomorrow so the next toolchain should hopefully have a workin REPL [and lldb].
I'll also try to add an integration test.