[SILGen] Don't emit executor checks for @_unsafeMainActor closures.

Narrow fix for rdar://76860623.

Author: DougGregor

lib/SILGen/SILGenProlog.cpp

@@ -524,7 +524,10 @@ void SILGenFunction::emitProlog(CaptureInfo captureInfo,
       break;
     }
   } else if (auto *closureExpr = dyn_cast<AbstractClosureExpr>(FunctionDC)) {
-    bool wantExecutor = F.isAsync() || !isInActorDestructor(closureExpr);
+    bool wantExecutor = F.isAsync() ||
+      (!isInActorDestructor(closureExpr) &&
+       !(isa<ClosureExpr>(closureExpr) &&
+         cast<ClosureExpr>(closureExpr)->isUnsafeMainActor()));
     auto actorIsolation = closureExpr->getActorIsolation();
     switch (actorIsolation.getKind()) {
     case ClosureActorIsolation::Independent:

test/SILGen/check_executor.swift

@@ -15,6 +15,8 @@ import _Concurrency
 
 func takeClosure(_ fn: @escaping () -> Int) { }
 
+func takeUnsafeMainActorClosure(@_unsafeMainActor _ fn: @escaping () -> Int) { }
+
 public actor MyActor {
   var counter = 0
 
@@ -36,4 +38,15 @@ public actor MyActor {
   deinit {
     takeClosure { self.counter }
   }
+
+  // CHECK-RAW-LABEL: sil private [ossa] @$s4test7MyActorC0A10UnsafeMainyyFSiycfU_
+  // CHECK-RAW-NOT: _checkExpectedExecutor
+  // CHECK-RAW: onMainActor
+  // CHECK-RAW: return
+  public func testUnsafeMain() {
+    takeUnsafeMainActorClosure {
+      onMainActor()
+      return 5
+    }
+  }
 }