Support for async/await, SE-0296

[jtbandes]

https://github.com/apple/swift-evolution/blob/main/proposals/0296-async-await.md

func foo() async {
  let x = await y
  let z = async  // async is only a contextual keyword
  let newURL = await server.redirectURL(for: url)
  let (data, response) = try await session.dataTask(with: newURL)
  let (data, response) = await try session.dataTask(with: newURL) // not allowed
  let (data, response) = await (try session.dataTask(with: newURL)) // ok
}
func foo() async -> Int {}
func foo() async throws -> (Int, String) {}
func foo() throws async -> (Int, String) {}
func foo() async rethrows {}
func foo() rethrows async {}
init() throws async {}
struct FunctionTypes {
  var syncNonThrowing: () -> Void
  var syncThrowing: () throws -> Void
  var asyncNonThrowing: () async -> Void
  var asyncThrowing: () async throws -> Void = x
}
let closure = { _ = await getInt() } // implicitly async
let closure = { (x: Int) async -> Int in 42 } // explicitly async
let closure = { (x: Int) throws -> Int in 42 }
let closure = { (x: Int) rethrows -> Int in 42 }
let closure = { (x: Int) async throws -> Int in 42 }
let closure = { (x: Int) throws async -> Int in 42 }