Use FilePath in VisualStudioWorkspaceImpl.AddDocumentToProjectItems

AddDocumentToProjectItems accepts a filePath argument but none of its
callers pass one, even though there's one on DocumentInfo.  If a
filePath is provided, don't attempt to retrieve the FULL_NAME from the
hierarchy to synthesize filePath.

Empirically, it looks like DocumentInfo.FilePath usually (always?)
returns null.

Context: we'd like to pass a non-null value in TypeScript as part of our
Move-to-File implementation.

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioWorkspaceImpl.cs

@@ -554,15 +554,15 @@ private void AddDocumentCore(DocumentInfo info, SourceText initialText, bool isA
 
             if (IsWebsite(project))
             {
-                AddDocumentToFolder(hostProject, project, info.Id, SpecializedCollections.SingletonEnumerable(AppCodeFolderName), info.Name, info.SourceCodeKind, initialText, isAdditionalDocument: isAdditionalDocument);
+                AddDocumentToFolder(hostProject, project, info.Id, SpecializedCollections.SingletonEnumerable(AppCodeFolderName), info.Name, info.SourceCodeKind, initialText, isAdditionalDocument: isAdditionalDocument, filePath: info.FilePath);
             }
             else if (folders.Any())
             {
-                AddDocumentToFolder(hostProject, project, info.Id, folders, info.Name, info.SourceCodeKind, initialText, isAdditionalDocument: isAdditionalDocument);
+                AddDocumentToFolder(hostProject, project, info.Id, folders, info.Name, info.SourceCodeKind, initialText, isAdditionalDocument: isAdditionalDocument, filePath: info.FilePath);
             }
             else
             {
-                AddDocumentToProject(hostProject, project, info.Id, info.Name, info.SourceCodeKind, initialText, isAdditionalDocument: isAdditionalDocument);
+                AddDocumentToProject(hostProject, project, info.Id, info.Name, info.SourceCodeKind, initialText, isAdditionalDocument: isAdditionalDocument, filePath: info.FilePath);
             }
 
             var undoManager = TryGetUndoManager();
@@ -644,7 +644,8 @@ protected override void AddExistingDocument(DocumentId documentId, string filePa
             string filePath = null,
             bool isAdditionalDocument = false)
         {
-            if (!project.TryGetFullPath(out var folderPath))
+            string folderPath = null;
+            if (filePath == null && !project.TryGetFullPath(out folderPath))
             {
                 // TODO(cyrusn): Throw an appropriate exception here.
                 throw new Exception(ServicesVSResources.Could_not_find_location_of_folder_on_disk);
@@ -665,7 +666,9 @@ protected override void AddExistingDocument(DocumentId documentId, string filePa
             bool isAdditionalDocument = false)
         {
             var folder = project.FindOrCreateFolder(folders);
-            if (!folder.TryGetFullPath(out var folderPath))
+
+            string folderPath = null;
+            if (filePath == null && !folder.TryGetFullPath(out folderPath))
             {
                 // TODO(cyrusn): Throw an appropriate exception here.
                 throw new Exception(ServicesVSResources.Could_not_find_location_of_folder_on_disk);