Array expression trailing closures

[calda]

Corresponding compiler PR: swiftlang/swift#86244

---

This PR prototypes support for [Element] { ... } trailing closure init syntax for arrays and dictionaries.

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.

[calda]

These now parse, but fail type checking in the full compiler:

  _ = [1] { return [1] } // expected-error {{cannot call value of non-function type '[Int]'}}
  _ = [1: 1] { return [1: 1] } // expected-error {{cannot call value of non-function type '[Int : Int]'}}

  _ = [1] // expected-error {{cannot call value of non-function type '[Int]'}}
  { return [1] }

  _ = [1: 1] // expected-error {{cannot call value of non-function type '[Int : Int]'}}
  { return [1: 1] }