Utility Analyzer: Decouple file I/O from Roslyn threads

[martin-strecker-sonarsource]

The Roslyn tasks that execute our analyzers expect that the logic is purely CPU-bound. It is highly discouraged to do any I/O in analyzers and source generators (citation needed).
The utility analyzer write to disk in the compilation end step and therefore performing I/O and block the analyzer thread. We should

• Perform the file I/O on a dedicated thread (not under Roslyns control)
• Start the file I/O as soon as the data is available and not only in the end step
• Do as little blocking as necessary in the end step to wait for the I/O thread to finish it's remaining work.