When building compiledb, we generate all the source codes, so any consumer of compile_commands.json will have any source needed to use the commands. However we use --build_tag_filters and "..." wildcard. The gen sources for the transition target are not included in this wild card and filter combo, as the transition only takes affect if the transition target itself is requested.

We worked around this issue in SERVER-109614 by just building the extension target directly but this is inefficient. Instead we should figure out how to generate the transition target sources.