C#: Add overlay builtins to the dbscheme.

csharp/ql/lib/semmlecode.csharp.dbscheme

@@ -194,6 +194,24 @@ externalData(
 sourceLocationPrefix(
   string prefix: string ref);
 
+/*
+ * Overlay support
+ */
+
+/**
+ * The CLI will automatically emit the tuple `databaseMetadata("isOverlay", "true")`,
+ * along with an `overlayChangedFiles` tuple for each new/modified/deleted file,
+ * when building an overlay database, and these can be used by the discard predicates.
+ */
+databaseMetadata(
+  string metadataKey : string ref,
+  string value : string ref
+);
+
+overlayChangedFiles(
+  string path : string ref
+);
+
 /*
  * C# dbscheme
  */