[generator] Don't re-add members as 'virtual' or 'new virtual' when conforming to protocol

[VincentDondain]

Steps to Reproduce

CNKeyDescriptor conforms to NSObjectProtocol and when I added CNKeyDescriptor conformance to NSString it re-decalred a bunch of members that were already there (from NSObject).

See generate code.

Expected Behavior

The generator should be smart enough to not re-generate members that are already defined in the base type.

Actual Behavior

Code is duplicated.

Environment

=== Visual Studio Community 2017 for Mac (Preview) ===

Version 7.4 Preview (7.4 build 720)
Installation UUID: 00d602ef-271b-479b-9054-0433b5797215
Runtime:
	Mono 5.8.0.99 (2017-10/e65bf00e22b) (64-bit)
	GTK+ 2.24.23 (Raleigh theme)

	Package version: 508000099

=== NuGet ===

Version: 4.3.1.4445

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	2.0.0-preview2-25407-01
	1.1.2
	1.0.5
SDK: /usr/local/share/dotnet/sdk/2.0.0-preview2-006497/Sdks
SDK Versions:
	2.0.0-preview2-006497
	1.0.4
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.8.0/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.5.6
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Apple Developer Tools ===

Xcode 9.2 (13772)
Build 9C40b

=== Xamarin.Mac ===

Version: 4.3.0.17 (Visual Studio Community)

=== Xamarin.iOS ===

Version: 11.9.0.17 (Visual Studio Community)
Hash: 57ef7c4d
Branch: bug-59272
Build date: 2018-01-12 13:44:41-0500

=== Xamarin.Android ===

Version: 8.0.2.1 (Visual Studio Community)
Android SDK: /Users/vidondai/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		6.0 (API level 23)
		7.1 (API level 25)
		8.0 (API level 26)

SDK Tools Version: 25.2.5
SDK Platform Tools Version: 25.0.4
SDK Build Tools Version: 23.0.2

Java SDK: /usr
java version "1.8.0_101"
Java(TM) SE Runtime Environment (build 1.8.0_101-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.101-b13, mixed mode)

Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Xamarin Inspector ===

Version: 1.3.2
Hash: 461f09a
Branch: 1.3-release
Build date: Tue, 03 Oct 2017 18:26:57 GMT
Client compatibility: 1

=== Build Information ===

Release ID: 704000720
Git revision: d5d46cb994643f64cc683737ecef76f581cf2b89
Build date: 2017-11-28 09:24:00-05
Xamarin addins: e7e3c0e88c36edf0498dba75a52e3c0654fe69ed
Build lane: monodevelop-lion-master

=== Operating System ===

Mac OS X 10.12.6
Darwin 16.7.0 Darwin Kernel Version 16.7.0
    Wed Oct  4 00:17:00 PDT 2017
    root:xnu-3789.71.6~1/RELEASE_X86_64 x86_64

=== Enabled user installed addins ===

NuGet Package Explorer 0.2
Internet of Things (IoT) development (Preview) 7.1

[spouliot]

To be more accurate: source code is duplicated, but binary-wise those duplicated methods ends up being new virtual which could cause hard to diagnose problems when subclassing NSString