Introduce `FixIt.Change` API that replaces the child of a syntax node

[ahoppen]

Currently, Fix-Its are only able to replace syntax nodes but that makes it impossible to replace a nil node by a proper syntax node.

We should offer a FixIt.Change that replaces the child of a syntax node, with a signature that’s something like this (written in GitHub, not sure if it compiles, but it gets the idea across).

FixIt.Change.replaceChild<Parent, Child>(_: KeyPath<Parent, Child>, of parent: Parent, with: Child)

FixItApplier will need to be updated to also handle this new change kind

rdar://115562892

[mininny]

I'd like to have a go at this if it's okay!

[kimdv]

@mininny it now yours :D

[AppAppWorks]

@ahoppen, I wanted to tackle this issue but apparently there's no way to make WritableKeyPath conform with Sendable 🤷‍♂️

[ahoppen]

You should be able to always use WritableKeyPath & Sendable, similar to

swift-syntax/Sources/SwiftSyntax/SyntaxNodeStructure.swift

Line 21 in d181e99

case layout([AnyKeyPath & Sendable])

[AppAppWorks]

What could be the use cases for replaceChild?

[j-f1]

One use case (that I ran into recently) is adding a return type to a function declaration that doesn’t have one. Or potentially appending/removing a parameter.