C#: Add internal queries for extractor and compiler diagnostics.

csharp/ql/src/Diagnostics/CompilerError.ql

@@ -0,0 +1,17 @@
+/**
+ * @name Compilation error
+ * @description A compilation error can lead to extraction problems, and may indicate
+ *              a bug in the extractor.
+ * @kind problem
+ * @problem.severity recommendation
+ * @precision medium
+ * @id cs/compilation-error
+ * @tags internal
+ */
+
+import csharp
+import semmle.code.csharp.commons.Diagnostics
+
+from CompilerError diagnostic
+select diagnostic,
+  diagnostic.getSeverityText() + " " + diagnostic.getTag() + " " + diagnostic.getFullMessage()