[Concurrency] Allow conditionally conforming to Sendable when conformance is suppressed

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!
```

test/ModuleInterface/tilde_sendable.swift

@@ -33,3 +33,18 @@ protocol P {
 public struct S: P, ~Sendable {
   public let x: Int
 }
+
+// CHECK: #if compiler(>=5.3) && $TildeSendable
+// CHECK: public struct B<T> : ~Swift.Sendable {
+// CHECK: }
+// CHECK: #else
+// CHECK: public struct B<T> {
+// CHECK: }
+// CHECK: #endif
+public struct B<T>: ~Sendable {
+}
+
+// CHECK: extension Library.B : Swift.Sendable where T : Swift.Sendable {
+// CHECK: }
+extension B: Sendable where T: Sendable {
+}