[clangd] code model lacks context for unity build sources

there is a small issue in the clangd code model when using cmake's unity builds: if a source file is opened in qtc which is part of a unity build, this source file is not parsed in the context of the corresponding cmake target. the "C++ Code Model Inspector" has a "Project Parts" entry. this entry is empty when i enable unity builds.

the expected behaviour would be to parse these sources in the context of the corresponding cmake target