Ensure we don't assume that the TU-file is the file we are working on.
This is done by explicitly requesting a path to be passed to ParseSession::file and adding ::mainFile with the old logic. This probably will fix quite some potential issues, e.g. in the signature assistant or code completion. Furthermore, we should not triviall be able to share the parse session between different contexts, if I'm not mistaken. REVIEW: 122094
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