Support array expression trailing closures

[calda]

Corresponding Swift Syntax PR: swiftlang/swift-syntax#3223

Swift evolution pitch: https://forums.swift.org/t/support-trailing-closure-syntax-for-single-argument-array-and-dictionary-initializers/83900

---

This PR prototypes support for array and dictionary expression trailing closure syntax.

Today, if you have an @ArrayBuilder type and define:

extension Array {
  init(@ArrayBuilder<Element> _ build: () -> [Element]) {
    self = build()
  }
}

you currently cannot call it like this:

let myArray = [String] { // error: 'let' declarations cannot be computed properties
  "foo"
  "bar"
}

This PR enables support for that syntax by allowing trailing closures following array literals.

[vanvoorden]

@calda Want to try a toolchain build for engineers to experiment with?

[calda]

That would be great. I would need somebody to trigger the build, since I don't have write access.

[vanvoorden]

@swift-ci Please Build Toolchain macOS Platform

[calda]

@vanvoorden, could you also run the source compatibility tests? Thanks!

[vanvoorden]

@swift-ci Please Test Source Compatibility Debug