Improve diagnostic when attempting to pattern match on `static func`

For e.g:

```swift
struct S {
  static func foo(_ x: Int) -> S { .init() }
}

func bar(_ x: S) {
  switch x {
  case .foo(let x): // error: 'let' binding pattern cannot appear in an expression
    ()
  }
}
```

We ought to be able to improve the diagnostic to point out that you're trying to pattern match a function argument rather than an enum case. 