Log source mappings

Found this lacking when I was trying to repro the issue, and thought it was in the C# formatter itself.

Author: davidwengier

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Formatting/Passes/CSharpFormattingPass.cs

@@ -28,6 +28,7 @@ public async Task<ImmutableArray<TextChange>> ExecuteAsync(FormattingContext con
         // Process changes from previous passes
         var changedText = context.SourceText.WithChanges(changes);
         var changedContext = await context.WithTextAsync(changedText, cancellationToken).ConfigureAwait(false);
+        context.Logger?.LogObject("SourceMappings", changedContext.CodeDocument.GetRequiredCSharpDocument().SourceMappings);
 
         // To format C# code we generate a C# document that represents the indentation semantics the user would be
         // expecting in their Razor file. See the doc comments on CSharpDocumentGenerator for more info