[Concurrency] Classes nested in actors are not semantically final.

The check for actor methods in `isSemanticallyFinal` was accidentally
kicking in for class members, which avoided `Sendable` checking on
classes nested in actors.

Author: hborla

lib/AST/Decl.cpp

@@ -3701,10 +3701,12 @@ bool ValueDecl::isSemanticallyFinal() const {
       return true;
   }
 
-  // As are members of actor types.
-  if (auto classDecl = getDeclContext()->getSelfClassDecl()) {
-    if (classDecl->isAnyActor())
-      return true;
+  // As are methods/accessors of actor types.
+  if (!isa<TypeDecl>(this)) {
+    if (auto classDecl = getDeclContext()->getSelfClassDecl()) {
+      if (classDecl->isAnyActor())
+        return true;
+    }
   }
 
   // For everything else, the same as 'final'.

test/Concurrency/sendable_conformance_checking.swift

@@ -181,3 +181,9 @@ extension SendableExtSub: @unchecked Sendable {}
 // Still want to know about same-class redundancy
 class MultiConformance: @unchecked Sendable {} // expected-note {{'MultiConformance' declares conformance to protocol 'Sendable' here}}
 extension MultiConformance: @unchecked Sendable {} // expected-error {{redundant conformance of 'MultiConformance' to protocol 'Sendable'}}
+
+@available(SwiftStdlib 5.1, *)
+actor MyActor {
+  // expected-warning@+1 {{non-final class 'Nested' cannot conform to 'Sendable'; use '@unchecked Sendable'; this is an error in the Swift 6 language mode}}
+  class Nested: Sendable {}
+}