Downgrade "reference to captured var" to warn in preconcurrency code.

Fixes rdar://97228088.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -2692,10 +2692,15 @@ namespace {
         }
 
         // Otherwise, we have concurrent access. Complain.
+        bool preconcurrencyContext =
+            getActorIsolationOfContext(
+              const_cast<DeclContext *>(getDeclContext())).preconcurrency();
+
         ctx.Diags.diagnose(
             loc, diag::concurrent_access_of_local_capture,
             parent.dyn_cast<LoadExpr *>(),
-            var->getDescriptiveKind(), var->getName());
+            var->getDescriptiveKind(), var->getName())
+          .warnUntilSwiftVersionIf(preconcurrencyContext, 6);
         return true;
       }
 

test/ClangImporter/objc_async.swift

@@ -114,7 +114,7 @@ func testSendable(fn: () -> Void) {
 func testSendableInAsync() async {
   var x = 17
   doSomethingConcurrentlyButUnsafe {
-    x = 42 // expected-error{{mutation of captured var 'x' in concurrently-executing code}}
+    x = 42 // expected-warning{{mutation of captured var 'x' in concurrently-executing code}}
   }
   print(x)
 }

test/Concurrency/concurrent_value_checking.swift

@@ -174,15 +174,15 @@ func testConcurrency() {
 func testUnsafeSendableNothing() {
   var x = 5
   acceptUnsafeSendable {
-    x = 17 // expected-error{{mutation of captured var 'x' in concurrently-executing code}}
+    x = 17 // expected-warning{{mutation of captured var 'x' in concurrently-executing code}}
   }
   print(x)
 }
 
 func testUnsafeSendableInAsync() async {
   var x = 5
   acceptUnsafeSendable {
-    x = 17 // expected-error{{mutation of captured var 'x' in concurrently-executing code}}
+    x = 17 // expected-warning{{mutation of captured var 'x' in concurrently-executing code}}
   }
   print(x)
 }