Fix diagnostic for `@c enum T: Int128`. (#90028)
This PR fixes the incorrect diagnostic when an integer type known to
Swift but not to C/Objective-C is used with an `@c` or `@objc` enum:
```swift
@c enum T: Int128 {} // 🛑 '@c' enum raw type 'Int128' is not an integer type
```
The diagnostic now reads:
> 🛑 '@c' enum raw type 'Int128' must be an integer type expressible in C
(Substituting "Objective-C" when `@objc` is used.)
