[Concurrency] Account for explicit @concurrent on closures

If closure is stated as `@concurrent` is has to be treated as
nonisolated.

Resolves: rdar://150397825

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -4681,6 +4681,10 @@ ActorIsolation ActorIsolationChecker::determineClosureIsolation(
           attr && ctx.LangOpts.hasFeature(Feature::ClosureIsolation)) {
         return ActorIsolation::forNonisolated(attr->isUnsafe());
       }
+
+      if (explicitClosure->getAttrs().hasAttribute<ConcurrentAttr>()) {
+        return ActorIsolation::forNonisolated(/*unsafe=*/false);
+      }
     }
 
     // `nonisolated(nonsending)` inferred from the context makes

test/Concurrency/attr_execution/conversions_silgen.swift

@@ -456,3 +456,15 @@ func testThatClosuresAssumeIsolation(fn: inout nonisolated(nonsending) (Int) asy
   // CHECK: hop_to_executor [[GENERIC_EXECUTOR]]
   fn = { @concurrent _ in }
 }
+
+@MainActor
+func testNoIsolationTransfer() {
+  // CHECK: // Isolation: global_actor. type: MainActor
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen23testNoIsolationTransferyyF0D7ErasureL_yyyyYaYAcF : $@convention(thin) (@guaranteed @isolated(any) @async @callee_guaranteed () -> ()) -> ()
+  func testErasure(@_inheritActorContext _: @escaping @isolated(any) () async -> Void) {}
+
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen23testNoIsolationTransferyyFyyYacfU_ : $@convention(thin) @async (@guaranteed Optional<any Actor>) -> ()
+  // CHECK: [[GENERIC_EXECUTOR:%.*]] = enum $Optional<Builtin.Executor>, #Optional.none!enumelt
+  // CHECK: hop_to_executor [[GENERIC_EXECUTOR]]
+  testErasure { @concurrent in }
+}