C#: Add extraction error diagnostic query

csharp/ql/src/Diagnostics/DiagnosticExtractionErrors.ql

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+/**
+ * @name Extraction errors
+ * @description List all errors reported by the extractor. The returned issues are
+ *              limited to those files where there are no compilation errors. This
+ *              indicates a bug or limitation in the extractor, and could lead to
+ *              inaccurate results.
+ * @kind diagnostic
+ * @id cs/diagnostics/extraction-errors
+ */
+
+import csharp
+import semmle.code.csharp.commons.Diagnostics
+
+from ExtractorError error
+where not exists(CompilerError ce | ce.getLocation().getFile() = error.getLocation().getFile())
+select error,
+  "Unexpected " + error.getOrigin() + " error: " + error.getText() + " in " +
+    error.getLocation().getFile(), 3

csharp/ql/test/library-tests/diagnostics/DiagnosticExtractorErrors.qlref

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+Diagnostics/DiagnosticExtractionErrors.ql

csharp/ql/test/library-tests/diagnostics/Program.cs

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+// semmle-extractor-options: --standalone
+
+using System;
+
+class Class
+{
+    static void Main(string[] args)
+    {
+        int z = GetParamLength(__arglist(1, 2));
+    }
+
+    public static int GetParamLength(__arglist)
+    {
+        ArgIterator iterator = new ArgIterator(__arglist);
+        return iterator.GetRemainingCount();
+    }
+}