Rewrite `build-script.py` in Swift

[ahoppen]

build-script.py is the only remaining Python file in SwiftSyntax. It would be nice to rewrite it in Swift so the only language used in SwiftSyntax is Swift (+ a tiny bit of C in Sources/_InstructionCounter).

[ahoppen]

rdar://107372272

[kimdv]

I have been thinking a bit here.
What do you think of we make a "swift-syntax-build-cli" package, just like code gen.
It uses the argument parser and have all the needed functionality as build-script.py

Then we move all the code gen into this package.

This package will in the future contain all the tooling needed for swift syntax. Formatting, test, code gen, verify code and so on.

[ahoppen]

Yes, that was essentially my idea. I think it might still be worthwhile to keep CodeGeneration as a separate executable just so you don’t need to build swift-syntax to run the newly written build-script.