Failed to produce diagnostic for invalid metatype to existential metatype conversion

The following fails to produce a diagnostic:

```swift
protocol P {}

func bar<T>(_ x: T) -> T.Type {}
func foo(_ x: P.Type) { }
foo(bar(0)) // error: failed to produce diagnostic for expression; please submit a bug report
```

It ought to produce the same diagnostic as:

```swift
foo(Int.self) // error: cannot convert value of type 'Int.Type' to expected argument type 'any P.Type'
```