[Sema/SILGen/IRGen] Implement method & initializer keypaths.

[amritpan]

This is the implementation for Method and Initializer Keypaths which extends keypath usage to include references to methods and initializers:

struct S {
  static let millenium = 3
  var year = 2024
  init() {}
  init(val value: Int = 2024) { year = value }
  
  func add(this: Int) -> Int { this + this}
  func add(that: Int) -> Int { that + that }
  static func subtract(_ val: Int) -> Int { return millenium - val }
  nonisolated func nonisolatedNextYear() -> Int { return year + 1 }
  consuming func consume() { print(year) }

  subscript(index: Int) -> Int { return year + index }
}

let kp1: KeyPath<S, () -> S> = \S.Type.init
let kp2: KeyPath<S, S> = \S.Type.init()
let kp3: KeyPath<S, (Int) -> S> = \S.Type.init(val:)
let kp4: KeyPath<S, S> = \S.Type.init(val: 2025)
let kp5: KeyPath<S, (Int) -> Int> = \S.add(this:)
let kp6: KeyPath<S, Int> = \S.add(that: 1)
let kp7: KeyPath<S, Int> = \S.Type.subtract(1)
let kp8: KeyPath<S, Int> = \S.nonisolatedNextYear()
let kp9: KeyPath<S, Int> = \S.nonisolatedNextYear().signum()
let kp10: KeyPath<S, String> = \S.nonisolatedNextYear().description
let kp11: KeyPath<S, ()> = \S.consume()
let kp12: KeyPath<S, Int> = \S.Type.init()[1]

inout, mutating, throws, and async methods are not allowed for existing keypath component types and similarly not supported by this implementation. escaping is also unsupported as keypath method arguments must be Hashable.

This implementation is accompanied by changes in swift-syntax and foundation.

[amritpan]

swiftlang/swift-syntax#2950

@swift-ci Please Test Source Compatibility.

[amritpan]

@swift-ci Please test.

[amritpan]

@swift-ci Please test.