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/BUILD.bazel

@@ -0,0 +1,8 @@
+py_binary(
+    name = "codegen",
+    srcs = glob(["**/*.py"]),
+    data = glob(["**/*.mustache"]) + [
+        "schema.yml",
+        "prefix.dbscheme",
+    ],
+)