Swift: dbscheme generator

This patch introduces the basic infrastructure of the code generation
suite and the `dbscheme` generator.

Notice that the checked in `schema.yml` should reflect swift 5.6 but
might need some tweaking.

Closes https://github.com/github/codeql-c-team/issues/979

swift/codegen/codegen.py

@@ -0,0 +1,7 @@
+#!/usr/bin/env python3
+
+from lib import generator
+import dbschemegen
+
+if __name__ == "__main__":
+    generator.run(dbschemegen.generate)