[Concurrency] Allow capturing nonisolated(unsafe) local variables in

`@Sendable` closures.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -2468,6 +2468,15 @@ namespace {
         // If the closure won't execute concurrently with the context in
         // which the declaration occurred, it's okay.
         auto decl = capture.getDecl();
+        auto isolation = getActorIsolation(decl);
+
+        // 'nonisolated' local variables are always okay to capture in
+        // 'Sendable' closures because they can be accessed from anywhere.
+        // Note that only 'nonisolated(unsafe)' can be applied to local
+        // variables.
+        if (isolation.isNonisolated())
+          continue;
+
         auto *context = localFunc.getAsDeclContext();
         auto fnType = localFunc.getType()->getAs<AnyFunctionType>();
         if (!mayExecuteConcurrentlyWith(context, decl->getDeclContext()))

test/Concurrency/sendable_checking.swift

@@ -367,4 +367,12 @@ final class UseNonisolatedUnsafe: Sendable {
   nonisolated(unsafe) var x1: NonSendable = .init()
   nonisolated(unsafe) let x2: NonSendable = .init()
   nonisolated(unsafe) var x3: Int = 0
+
+  func captureInTask() {
+    nonisolated(unsafe) var x = NonSendable()
+    Task {
+      print(x)
+      x = NonSendable()
+    }
+  }
 }