ClangImporter: Fix mirroring of instance properties as static methods on NSObject

lib/ClangImporter/ClangImporter.cpp

@@ -3787,6 +3787,17 @@ ClangImporter::Implementation::loadNamedMembers(
           Members.push_back(V);
         }
       }
+
+      // If the property's accessors have alternate decls, we might have
+      // to import those too.
+      if (auto *ASD = dyn_cast<AbstractStorageDecl>(TD)) {
+        for (auto *AD : ASD->getAllAccessors()) {
+          for (auto *D : getAlternateDecls(AD)) {
+            if (D->getBaseName() == N)
+              Members.push_back(D);
+          }
+        }
+      }
     }
   }
 

test/ClangImporter/mirror_instance_property_static_method.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-sil %s -verify
+// REQUIRES: objc_interop
+
+import Foundation
+
+func mirrorInstancePropertyAsStaticMethod() {
+  // Instance properties are mirrored as static _methods_. Make sure this works.
+  let _: AnyClass = NSObject.classForCoder()
+}

test/Inputs/clang-importer-sdk/usr/include/objc/NSObject.h

@@ -27,6 +27,10 @@
 @property (readonly) NSInteger hash;
 @end
 
+@interface NSObject (Coding)
+- (Class)classForCoder;
+@end
+
 @interface A : NSObject
 - (int)method:(int)arg withDouble:(double)d;
 + (int)classMethod;