Bootstrapping SwiftSyntaxBuilder

Providing excellent tooling for Swift often involves operating on Swift syntax trees directly. The swift-syntax library serves this purpose by providing a Swift representation of syntax trees, along with facilities to parse and generate Swift code. To make code generation especially convenient, the library includes a Domain-Specific Language (DSL) for building syntax trees that, however, currently relies on external tooling to generate a large part of its own definitions. To test the DSL on a non-trivial codebase and to make it production-ready, we will rewrite the syntax builder in itself, generating its sources using its own DSL and thereby bootstrapping its implementation.