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/lib/paths.py

@@ -0,0 +1,15 @@
+import pathlib
+import sys
+import os
+
+try:
+    _workspace_dir = pathlib.Path(os.environ['BUILD_WORKSPACE_DIRECTORY'])
+    swift_dir = _workspace_dir / 'swift'
+    lib_dir = swift_dir / 'codegen' / 'lib'
+except KeyError:
+    _this_file = pathlib.Path(__file__).resolve()
+    swift_dir = _this_file.parents[2]
+    lib_dir = _this_file.parent
+
+
+exe_file = pathlib.Path(sys.argv[0]).resolve()