Merge #7115

7115: Migrate HasSource::source to return Option r=matklad a=nick96

I've made a start on fixing #6913 based on the provided work plan, migrating `HasSource::source` to return an `Option`. The simple cases are migrated but there are a few that I'm unsure exactly how they should be handled:

- Logging the processing of functions in `AnalysisStatsCmd::run`: In verbose mode it includes the path to the module containing the function and the syntax range. I've handled this with an if-let but would it be better to blow up here with `expect`? I'm not 100% on the code paths but if we're processing a function definition then the source should exist.

I've handled `source()` in all code paths as `None` being a valid return value but are there some cases where we should just blow up? Also, all I've done is bubble up the returned `None`s, there may be some places where we can recover and still provide something.

Co-authored-by: Nick Spain <nicholas.spain@stileeducation.com>
Co-authored-by: Nick Spain <nicholas.spain96@gmail.com>

diff --cc crates/assists/src/utils.rs
Simple merge