Swift IR tree generator

This directory contains a code generator for generating Swift IR classes. We reuse the infrastructure for tree generators that is also used for generating IR and FIR trees.

• All tree elements are declared in SwiftIrTree.kt
• Types commonly used in configuration are listed in Types.kt
• If an element has no inheritors, then it will have a default implementation. Otherwise, you should declare an implementation that you want in ImplementationConfigurator.kt.
• The same is true for builders. For each leaf element in the hierarchy, there is a corresponding builder class.
• Builders are configured in BuilderConfigurator.kt.