Tests can be run with
```
bazel test //swift/codegen:tests
```
Coverage can be checked installing `pytest-cov` and running
```
pytest --cov=swift/codegen swift/codegen/test
```
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
This adds a first dummy extractor for swift.
Running `bazel run //swift:install` will create an `extractor_pack`
directory in `swift`. From that moment providing `--search-path=swift`
will pick up the extractor.
