[SourceLink] Output window logging

[davidwengier]

It'd be useful to log to output window when we encounter errors, so that if something goes wrong the customer can inspect the output and see why it didn't work.

Originally posted by @tmat in #57073 (comment)

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This should apply more broadly too, and we can log where we look for pdbs, source files, etc.

[sharwell]

We have a Roslyn output window for this right?

[davidwengier]

We have a Roslyn output window for this right?

Had a quick look at the roslyn output window we have is part of the logging infrastructure, and we'd want to keep this limited to Go To Def. I can experiment with enabling it only while GTD is running, but it might get noise from other things we don't want.

The aim for logging here is to be user facing to library authors diagnose why things might not be working.

[davidwengier]

Fixed by #58209