Allow `@_section` and `@_used` to be used in fully constrained extensions.
This aligns with the existing behavior of static properties of generic types. The following is valid:
```swift
extension Array where Element == Int {
static let specificConstant = 41
}
```
However, adding `@_section` or `@_used` to the above property, will result in an error diagnostic.
This change updates the logic of `@_section`/`@_used` to allow their use when the generic context is fully concrete.
- Drop the current requirement that these attributes can only apply to top-level declarations
- Diagnose if they are used in local contexts
- Diagnose if they are used in generic contexts
- Add tests