[ObjectiveC] Make NSObject.hashValue non-overridable

We deprecated overriding it in 4.2 with a custom compiler warning. This removes the ability to override it entirely.

The correct way to customize hashing for NSObject subclasses is to override the `hash` property.

test/stdlib/NSObject-hashing.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+// RUN: %target-swift-frontend -c -verify -verify-ignore-unknown %s -o /dev/null
+
+// REQUIRES: objc_interop
+
+import ObjectiveC
+
+class Foo: NSObject {
+  override var hashValue: Int { // expected-error {{overriding non-open property outside of its defining module}} expected-error {{overriding non-@objc declarations from extensions is not supported}}
+    return 0
+  }
+
+  override func hash(into hasher: inout Hasher) { // expected-error {{overriding non-open instance method outside of its defining module}} expected-error {{overriding declarations in extensions is not supported}}
+  }
+}