[Concurrency] Fix disallowed override isolation to carry @preconcurrency bit

If ObjC member cannot be overridden due to isolation mismatch set `@preconcurrency`
bit to make the diagnostic a warning instead of an error in Swift 5 mode.

Resolves: rdar://130776220
(cherry picked from commit f7d3230)

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -5274,7 +5274,12 @@ ActorIsolation ActorIsolationRequest::evaluate(
           break;
 
         case OverrideIsolationResult::Disallowed:
-          inferred = *overriddenIso;
+          if (overriddenValue->hasClangNode() &&
+              overriddenIso->isUnspecified()) {
+            inferred = overriddenIso->withPreconcurrency(true);
+          } else {
+            inferred = *overriddenIso;
+          }
           break;
         }
       }

test/ClangImporter/objc_isolation_complete.swift

@@ -31,3 +31,16 @@ class IsolatedSub: NXSender {
     return mainActorState
   }
 }
+
+@objc
+@MainActor
+class Test : NSObject {
+  static var shared: Test? // expected-note {{mutation of this static property is only permitted within the actor}}
+
+  override init() {
+    super.init()
+
+    Self.shared = self
+    // expected-warning@-1 {{main actor-isolated static property 'shared' can not be mutated from a nonisolated context; this is an error in the Swift 6 language mode}}
+  }
+}