Diagnose invalid @_hasStorage attributes

A bug in `@objc @implementation` is causing incorrect `@_hasStorage` attributes to be printed into module interfaces. As an initial step towards fixing this, diagnose bad `@_hasStorage` attributes and treat them as computed properties so that these malformed interfaces don’t cause compiler crashes.

Partially fixes rdar://144811653.

test/ModuleInterface/has_storage_attr_bad.swiftinterface

@@ -0,0 +1,10 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags:
+// expected-error@-2 {{failed to verify module interface of 'Test' due to the errors above}}
+
+// RUN: %target-swift-typecheck-module-from-interface(%s) -module-name Test -verify -verify-ignore-unknown
+// REQUIRES: OS=macosx
+
+extension Array {
+  @_hasStorage public var foo: Int { get set } // expected-error {{'@_hasStorage' attribute cannot be applied to declaration in extension}}
+}