Dan Zheng f44064cbbc [SE-0253] Introduce callables. (#24299) ...

Introduce callables: values of types that declare `func callAsFunction`
methods can be called like functions. The call syntax is shorthand for
applying `func callAsFunction` methods.

```swift
struct Adder {
  var base: Int
  func callAsFunction(_ x: Int) -> Int {
    return x + base
  }
}
var adder = Adder(base: 3)
adder(10) // desugars to `adder.callAsFunction(10)`
```

`func callAsFunction` argument labels are required at call sites.
Multiple `func callAsFunction` methods on a single type are supported.
`mutating func callAsFunction` is supported.

SR-11378 tracks improving `callAsFunction` diagnostics.

2019-08-26 23:56:36 -07:00

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