Use the CoreRT compiler from the new blob feed (#4617)

* Use CoreRT compiler from the new blob feed

See https://github.com/dotnet/corert/pull/7268

* Opt out of compiler defaults

These options became the defaults in the past months, but the benchmark doesn't need them. This removed almost 10 MB of cruft from the produced executable (which likely helps working set and code locality).

frameworks/CSharp/aspnetcore-corert/PlatformBenchmarks/NuGet.Config

@@ -2,7 +2,7 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
+    <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
     <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>

frameworks/CSharp/aspnetcore-corert/PlatformBenchmarks/PlatformBenchmarks.csproj

@@ -5,6 +5,11 @@
     <OutputType>Exe</OutputType>
     <LangVersion>latest</LangVersion>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+
+    <!-- Opt out of the "easy mode" of the CoreRT compiler (http://aka.ms/OptimizeCoreRT) -->
+    <RootAllApplicationAssemblies>false</RootAllApplicationAssemblies>
+    <IlcGenerateCompleteTypeMetadata>false</IlcGenerateCompleteTypeMetadata>
+    <IlcGenerateStackTraceData>false</IlcGenerateStackTraceData>
   </PropertyGroup>
   
   <ItemGroup>