Sema: Don't remove nonisolated attribute when we diagnose it as invalid on 'lazy'

Otherwise, we'll in turn complain if the nonisolated lazy property was
@objc. This is also invalid, but the goal here is to avoid the source
break until -swift-version 6.

Fixes rdar://141967932.

Author: slavapestov

lib/Sema/TypeCheckAttr.cpp

@@ -7449,7 +7449,7 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
     }
 
     if (var->getAttrs().hasAttribute<LazyAttr>()) {
-      diagnoseAndRemoveAttr(attr, diag::nonisolated_lazy)
+      diagnose(attr->getLocation(), diag::nonisolated_lazy)
         .warnUntilSwiftVersion(6);
       return;
     }

test/Concurrency/actor_isolation_objc.swift

@@ -64,3 +64,8 @@ actor Dril: NSObject {
 
 // makes sure the synthesized init's delegation kind is determined correctly.
 actor Pumpkin: NSObject {}
+
+actor Bad {
+  @objc nonisolated lazy var invalid = 0
+  // expected-warning@-1 {{'nonisolated' is not supported on lazy properties; this is an error in the Swift 6 language mode}}
+}