[PrintAsObjC] Test that we don't leak extended implementationOnly types

Author: xymus

test/PrintAsObjC/Inputs/custom-modules/MiserablePileOfSecrets.h

@@ -3,3 +3,6 @@
 @interface NSObject (Secrets)
 - (void)secretMethod;
 @end
+
+@interface SecretClass : NSObject
+@end

test/PrintAsObjC/imports.swift

@@ -77,3 +77,7 @@ import MostlyPrivate2_Private
 @objc public class TestSubclass: NSObject {
   @_implementationOnly public override func secretMethod() {}
 }
+
+extension SecretClass {
+  private func somethingThatShouldNotBeIncluded() {}
+}