Enable reference to prebuilt c# projects

vsintegration/src/FSharp.LanguageService/FSharp.LanguageService.fsproj

@@ -67,6 +67,7 @@
       <GeneratedModuleName>Microsoft.VisualStudio.FSharp.LanguageService.Strings</GeneratedModuleName>
     </EmbeddedResource>
     <Compile Include="AssemblyInfo.fs" />
+    <Compile Include="LanguageServiceConstants.fs" />
     <Compile Include="Error.fs" />
     <Compile Include="Vs.fs" />
     <Compile Include="Colorize.fs" />

vsintegration/src/FSharp.LanguageService/LanguageServiceConstants.fs

+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Microsoft.VisualStudio.FSharp.LanguageService
+
+[<RequireQualifiedAccess>]
+module internal LanguageServiceConstants =
+    
+    /// "F#"
+    [<Literal>]
+    let FSharpLanguageName = "F#"

vsintegration/src/FSharp.LanguageService/ProjectSitesAndFiles.fs

@@ -274,7 +274,7 @@ type internal ProjectSitesAndFiles() =
                         if not (isNull project) then
                             for reference in project.ProjectReferences do
                                 let project = workspace.CurrentSolution.GetProject(reference.ProjectId)
-                                if not (isNull project) then
+                                if not (isNull project) && (project.Language = LanguageServiceConstants.FSharpLanguageName) then
                                     let siteProvider = provideProjectSiteProvider (workspace, project, serviceProvider, projectOptionsTable)
                                     let referenceProject = workspace.ProjectTracker.GetProject(reference.ProjectId)
                                     let outputPath = referenceProject.BinOutputPath