[Concurrency] Don't warn about re-stating inherited unavailable conformances

to `Sendable`.

The unavailability check was not using the root conformance, which is where
the extension declaration with the unavailability attribute is for inherited
conformances, leading to bogus warnings about re-stating unchecked conformances
to `Sendable`.

(cherry picked from commit 69b2435)

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -6186,7 +6186,11 @@ bool swift::checkSendableConformance(
     return false;
 
   // If this is an always-unavailable conformance, there's nothing to check.
-  if (auto ext = dyn_cast<ExtensionDecl>(conformanceDC)) {
+  // We always use the root conformance for this check, because inherited
+  // conformances need to walk back to the original declaration for the
+  // superclass conformance to find an unavailable attribute.
+  if (auto ext = dyn_cast<ExtensionDecl>(
+          conformance->getRootConformance()->getDeclContext())) {
     if (AvailableAttr::isUnavailable(ext))
       return false;
   }

test/Concurrency/concurrent_value_checking.swift

@@ -380,6 +380,14 @@ final class C7<T>: Sendable { }
 
 class C9: Sendable { } // expected-warning{{non-final class 'C9' cannot conform to 'Sendable'; use '@unchecked Sendable'}}
 
+@available(*, unavailable)
+extension HasUnavailableSendable : @unchecked Sendable { }
+
+class HasUnavailableSendable {
+}
+
+class NoRestated: HasUnavailableSendable {} // okay
+
 @globalActor
 struct SomeActor {
   static let shared = A1()