Actor isolation inference: @IBAction implies @mainactor(unsafe)

@IBAction calls are always delivered in the main thread, so reflect that
by inferring `@MainActor(unsafe)` for such methods whenever no other
actor isolation is provided.

Implements rdar://84474640.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -3435,6 +3435,15 @@ ActorIsolation ActorIsolationRequest::evaluate(
     }
   }
 
+  // @IBAction implies @MainActor(unsafe).
+  if (value->getAttrs().hasAttribute<IBActionAttr>()) {
+    ASTContext &ctx = value->getASTContext();
+    if (Type mainActor = ctx.getMainActorType()) {
+      return inferredIsolation(
+          ActorIsolation::forGlobalActor(mainActor, /*unsafe=*/true));
+    }
+  }
+
   // Default isolation for this member.
   return defaultIsolation;
 }

test/Concurrency/global_actor_inference.swift

@@ -517,6 +517,15 @@ func useUnsafeMainActor() {
   }
 }
 
+// ----------------------------------------------------------------------
+// @IBAction implies @MainActor(unsafe)
+// ----------------------------------------------------------------------
+class SomeWidgetThing {
+  @IBAction func onTouch(_ object: AnyObject) {
+    onlyOnMainActor() // okay
+  }
+}
+
 // ----------------------------------------------------------------------
 // @_inheritActorContext
 // ----------------------------------------------------------------------