C++: Add databaseMetadata and overlayChangedFiles tables to dbscheme

Author: igfoo

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -1,3 +1,26 @@
+
+/*- Database metadata -*/
+
+/**
+ * The CLI will automatically emit applicable tuples for this table,
+ * such as `databaseMetadata("isOverlay", "true")` when building an
+ * overlay database.
+ */
+databaseMetadata(
+  string metadataKey: string ref,
+  string value: string ref
+);
+
+/*- Overlay support -*/
+
+/**
+ * The CLI will automatically emit tuples for each new/modified/deleted file
+ * when building an overlay database.
+ */
+overlayChangedFiles(
+  string path: string ref
+);
+
 /*- Compilations -*/
 
 /**