C#: Change the populate logic context. It looks like a mistake that t…

…he only flag set is Standalone.

csharp/extractor/Semmle.Extraction.CSharp/Extractor/Context.cs

@@ -267,7 +267,7 @@ private void Populate(ISymbol? optionalSymbol, Entities.CachedEntity entity)
 
             bool duplicationGuard, deferred;
 
-            if (ExtractionContext.Mode is ExtractorMode.Standalone)
+            if (ExtractionContext.IsStandalone)
             {
                 duplicationGuard = false;
                 deferred = false;