Don't diagnose non-Sendable captures in actor-isolated @sendable closures

Another important step towards implementing SE-0338 semantics.
Fixes rdar://90950355.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -1475,9 +1475,6 @@ namespace {
 
     /// Check closure captures for Sendable violations.
     void checkClosureCaptures(AbstractClosureExpr *closure) {
-      if (!isSendableClosure(closure, /*forActorIsolation=*/false))
-        return;
-
       SmallVector<CapturedValue, 2> captures;
       closure->getCaptureInfo().getLocalCaptures(captures);
       for (const auto &capture : captures) {
@@ -1486,7 +1483,12 @@ namespace {
         if (capture.isOpaqueValue())
           continue;
 
+        // If the closure won't execute concurrently with the context in
+        // which the declaration occurred, it's okay.
         auto decl = capture.getDecl();
+        if (!mayExecuteConcurrentlyWith(closure, decl->getDeclContext()))
+          continue;
+
         Type type = getDeclContext()
             ->mapTypeIntoContext(decl->getInterfaceType())
             ->getReferenceStorageReferent();

test/Concurrency/actor_isolation.swift

@@ -1467,3 +1467,22 @@ class SGA_MADirect: MADirect {
   // directly-nonisolated vs overridden-MainActor = nonisolated
   nonisolated override func method2() { beets_ma() } // expected-error {{call to main actor-isolated global function 'beets_ma()' in a synchronous nonisolated context}}
 }
+
+// Actor isolation allows capture
+extension MyActor {
+  func testNonSendableCaptures(sc: SomeClass) {
+    Task {
+      _ = self
+      _ = sc
+
+      Task { [sc,self] in
+        _ = self
+        _ = sc
+
+        Task {
+          _ = sc // expected-warning{{capture of 'sc' with non-sendable type 'SomeClass' in a `@Sendable` closure}}
+        }
+      }
+    }
+  }
+}