[Concurrency] Apply nonisolated(unsafe) to iterator variables for

async for-in loops.

Async iterators are not `Sendable`; they're only meant to be used from
the isolation domain that creates them. But the `next()` method runs on
the generic executor, so calling it from an actor-isolated context passes
non-`Sendable` state across the isolation boundary. `next()` should
inherit the isolation of the caller, but for now, use the opt out.

(cherry picked from commit 0c6c369)

Author: hborla

lib/Sema/CSGen.cpp

@@ -4413,6 +4413,19 @@ generateForEachStmtConstraints(ConstraintSystem &cs,
               sequenceExpr->getStartLoc(), ctx.getIdentifier(name), dc);
   makeIteratorVar->setImplicit();
 
+  // FIXME: Apply `nonisolated(unsafe)` to async iterators.
+  //
+  // Async iterators are not `Sendable`; they're only meant to be used from
+  // the isolation domain that creates them. But the `next()` method runs on
+  // the generic executor, so calling it from an actor-isolated context passes
+  // non-`Sendable` state across the isolation boundary. `next()` should
+  // inherit the isolation of the caller, but for now, use the opt out.
+  if (isAsync) {
+    auto *nonisolated = new (ctx)
+        NonisolatedAttr(/*unsafe=*/true, /*implicit=*/true);
+    makeIteratorVar->getAttrs().add(nonisolated);
+  }
+
   // First, let's form a call from sequence to `.makeIterator()` and save
   // that in a special variable which is going to be used by SILGen.
   {

test/Concurrency/experimental_feature_strictconcurrency.swift

@@ -91,4 +91,8 @@ func iterate(stream: AsyncStream<Int>) async {
   while let element = await it.next() {
     print(element)
   }
+
+  for await x in stream {
+    print(x)
+  }
 }