Resolve Visual Studio build issues

Visual Studio defaults to a C# version of "latest major". Some of the new projects require C# 7.1 features.

ARMeilleure/ARMeilleure.csproj

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>netcoreapp2.1</TargetFramework>
     <RuntimeIdentifiers>win-x64;osx-x64;linux-x64</RuntimeIdentifiers>
+    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">

Ryujinx.Tests/Ryujinx.Tests.csproj

@@ -14,6 +14,7 @@
 
   <PropertyGroup>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
+    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Profile Release|AnyCPU'">