Build fails for metal shader on Mojave

### Steps to Reproduce

1. Create a new iOS project in VS for Mac
2. Add this metal shader: https://raw.githubusercontent.com/laanlabs/SCNTechniqueGlow/master/SCNTechniqueGlow/NodeRender.metal
3. Build for an iOS device

### Expected Behavior

It builds.

### Actual Behavior

Build fails with error: /Library/Frameworks/Mono.framework/External/xbuild/Xamarin/iOS/Xamarin.iOS.Common.targets(3,3): Error MSB6006: "metal" exited with code 1. (MSB6006)

Note: The shader builds fine in Xcode or with Metal's command line tools.

### Environment

```
=== Visual Studio Community 2017 for Mac ===

Version 7.6.6 (build 14)
Installation UUID: 2aa0e016-a24d-4d38-b7a1-1664ab263dd1
Runtime:
Mono 5.12.0.305 (2018-02/e0e035e3b14) (64-bit)
GTK+ 2.24.23 (Raleigh theme)
Xamarin.Mac 4.4.1.178 (master / eeaeb7e6)

Package version: 512000305

=== NuGet ===

Version: 4.3.1.4445

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
2.1.2
2.1.1
2.0.5
2.0.0
SDK: /usr/local/share/dotnet/sdk/2.1.302/Sdks
SDK Versions:
2.1.302
2.1.301
2.1.4
2.0.0
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.12.0/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.6.3
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Xamarin.Android ===

Version: 9.0.0.20 (Visual Studio Community)
Android SDK: /Users/mike/Library/Android/sdk
Supported Android versions:
6.0 (API level 23)
8.0 (API level 26)
8.1 (API level 27)

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 27.0.1
SDK Build Tools Version: 27.0.3

Java SDK: /usr
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

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

=== Apple Developer Tools ===

Xcode 10.0 (14320.25)
Build 10A255

=== Xamarin.Mac ===

Version: 4.99.3.741 (Visual Studio Community)
Hash: eeca0b87
Branch:
Build date: 2018-09-13 15:01:04-0400

=== Xamarin.iOS ===

Version: 12.0.0.11 (Visual Studio Community)
Hash: eeca0b87
Branch: xcode10
Build date: 2018-09-13 15:01:03-0400

=== Xamarin Inspector ===

Version: 1.4.3
Hash: db27525
Branch: 1.4-release
Build date: Mon, 09 Jul 2018 21:20:18 GMT
Client compatibility: 1

=== Build Information ===

Release ID: 706060014
Git revision: 5fb9d5d4381378b850aab3fb6c59252695f6776d
Build date: 2018-09-14 16:13:50+00
Build branch: release-7.6-xcode10
Xamarin extensions: 96db24c7730531168cd53408e452a6d95150e05b

=== Operating System ===

Mac OS X 10.14.0
Darwin 18.0.0 Darwin Kernel Version 18.0.0
Wed Aug 22 20:13:40 PDT 2018
root:xnu-4903.201.2~1/RELEASE_X86_64 x86_64

=== Enabled user installed extensions ===

Internet of Things (IoT) development (Preview) 7.1

```

### Build Logs

https://gist.github.com/mikebluestein/200d75a0b29b616dface410b3c2c1123

### Example Project (If Possible)

[test case.zip](https://github.com/xamarin/xamarin-macios/files/2446464/test.case.zip)