Dev15.6 -- Fix multi-targetting / F# IDE (dotnet#4286)

* Fix multi-targetting

* Refactor

* address project relative paths

Author: KevinRansom

vsintegration/src/FSharp.Editor/LanguageService/LanguageService.fs

@@ -226,13 +226,21 @@ type internal FSharpProjectOptionsManager
 
     [<Export>]
     /// This handles commandline change notifications from the Dotnet Project-system
+    /// Prior to VS 15.7 path contained path to project file, post 15.7 contains target binpath
+    /// binpath is more accurate because a project file can have multiple in memory projects based on configuration
     member this.HandleCommandLineChanges(path:string, sources:ImmutableArray<CommandLineSourceFile>, references:ImmutableArray<CommandLineReference>, options:ImmutableArray<string>) =
+        let projectId =
+            match workspace.ProjectTracker.TryGetProjectByBinPath(path) with
+            | true, project -> project.Id
+            | false, _ -> workspace.ProjectTracker.GetOrCreateProjectIdForPath(path, projectDisplayNameOf path)
+        let project =  workspace.ProjectTracker.GetProject(projectId)
+        let path = project.ProjectFilePath
         let fullPath p =
-            if Path.IsPathRooted(p) then p
+            if Path.IsPathRooted(p) || path = null then p
             else Path.Combine(Path.GetDirectoryName(path), p)
         let sourcePaths = sources |> Seq.map(fun s -> fullPath s.Path) |> Seq.toArray
         let referencePaths = references |> Seq.map(fun r -> fullPath r.Reference) |> Seq.toArray
-        let projectId = workspace.ProjectTracker.GetOrCreateProjectIdForPath(path, projectDisplayNameOf path)
+
         projectOptionsTable.SetOptionsWithProjectId(projectId, sourcePaths, referencePaths, options.ToArray())
         this.UpdateProjectInfoWithProjectId(projectId, "HandleCommandLineChanges", invalidateConfig=true)