Array expression trailing closures

[calda]

This PR adds a proposal to add support for trailing closures following array and dictionary literals. This enables trailing closure syntax for init(_:)s defined on Array and Dictionary.

• Swift Evolution pitch: https://forums.swift.org/t/support-trailing-closure-syntax-for-single-argument-array-and-dictionary-initializers/83900
• Implementation: Support array expression trailing closures swift#86244
• Toolchain: https://ci.swift.org/job/swift-PR-toolchain-macos/2171/

Example:

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

let value = [String] {
    "a"
    "b"
    "c"
}