For consistency with invertible protocols using `~Sendable` should
only prohibit use of unconditional extensions.
For example:
```swift
struct G<T>: ~Sendable {}
```
The following (unconditional) extension is rejected:
```
extension G: Sendable {} // error: cannot both conform to and suppress conformance to 'Sendable'
```
But conditional on `T` is accepted:
```
extension G: Sendable where T: Sendable {} // Ok!
```
