Add AssemblyMetadata[Language] attribute (#596)

* Add [assembly:AssemblyMetadata(Language, C++)] attribute to aid CrossGen

Author: vatsan-madhavan

eng/WpfArcadeSdk/tools/ExtendedAssemblyInfo.targets

@@ -40,6 +40,26 @@
       <AssemblyMetadata Include="Commit" Condition="'$(SourceRevisionId)'!=''">
         <Value>$(SourceRevisionId)</Value>
       </AssemblyMetadata>
+
+      <!-- 
+        Crossgen wants to detect when an assembly is built from C++/CLI sources, 
+        since it is unable to function correctly today (ostensibly due to bugs). Since
+        /clr:pure is deprecated and WPF will eventually switch over to /clr (i.e., IJW),
+        Crossgen will eventually (i.e., in the near-term) fail to work on C++/CLI 
+        assemblies anyway. 
+
+        Crossgen will detect [assembly: AssemblyMetadta("Language", "C++")] and
+        skip those assemblies. WPF will add this attribute to assemblies that are
+        compiled from C++/CLI sources. 
+
+        WPF will also add [assembly: AssemblyMetadata("Language, "C#")] to C#
+        assemblies.
+        
+        https://github.com/dotnet/wpf/issues/587
+      --> 
+      <AssemblyMetadata Include="Language" Condition="'$(Language)'!=''">
+        <Value>$(Language)</Value>
+      </AssemblyMetadata>
     </ItemGroup>
   </Target>