SA1412 "fix all" does not work properly on files that are closed

[sharwell]

The "Fix all in project" operation for SA1412 is currently only applying changes to files that are open in the editor. Presumably this is caused by dotnet/roslyn#4262, where it thinks all the files on disk are UTF-8 without byte order mark, but all files open in the editor are Windows-1252 (the default system encoding).

When I opened any single file violating this rule and used File → Advanced Save Options... to explicitly change the file to UTF-8 without byte order mark, it allowed me to apply the fix all operation to the entire solution. This means all of the equivalence keys from the project were using "Windows-1252" if the file was open, and "utf-8" if the file was closed.

[sharwell]

💭 It might be possible to fix this by passing the name of the detected encoding as an additional property of the diagnostic, and using that rather than probing the file again from within the code fix provider.