Add a Solution.AddAdditionalDocuments method

We only allowed adding one-at-at-time. This aligns us with
regular documents and AnalyzerConfig documents.

src/Workspaces/Core/Portable/PublicAPI.Unshipped.txt

 *REMOVED*Microsoft.CodeAnalysis.Workspace.ClearOpenDocument(Microsoft.CodeAnalysis.DocumentId documentId, bool isSolutionClosing = false) -> void
+Microsoft.CodeAnalysis.Solution.AddAdditionalDocuments(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.DocumentInfo> documentInfos) -> Microsoft.CodeAnalysis.Solution
 Microsoft.CodeAnalysis.Solution.AddAnalyzerConfigDocuments(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.DocumentInfo> documentInfos) -> Microsoft.CodeAnalysis.Solution
 Microsoft.CodeAnalysis.Solution.ContainsAnalyzerConfigDocument(Microsoft.CodeAnalysis.DocumentId documentId) -> bool
 Microsoft.CodeAnalysis.Solution.RemoveAnalyzerConfigDocument(Microsoft.CodeAnalysis.DocumentId documentId) -> Microsoft.CodeAnalysis.Solution

src/Workspaces/Core/Portable/Workspace/Solution/ProjectState.cs

@@ -675,14 +675,14 @@ public ProjectState AddDocuments(ImmutableArray<DocumentState> documents)
                 documentStates: _documentStates.AddRange(documents.Select(d => KeyValuePairUtil.Create(d.Id, d))));
         }
 
-        public ProjectState AddAdditionalDocument(TextDocumentState document)
+        public ProjectState AddAdditionalDocuments(ImmutableArray<TextDocumentState> documents)
         {
-            Debug.Assert(!this.AdditionalDocumentStates.ContainsKey(document.Id));
+            Debug.Assert(!documents.Any(d => this.AdditionalDocumentStates.ContainsKey(d.Id)));
 
             return this.With(
                 projectInfo: this.ProjectInfo.WithVersion(this.Version.GetNewerVersion()),
-                additionalDocumentIds: _additionalDocumentIds.Add(document.Id),
-                additionalDocumentStates: _additionalDocumentStates.Add(document.Id, document));
+                additionalDocumentIds: _additionalDocumentIds.AddRange(documents.Select(d => d.Id)),
+                additionalDocumentStates: _additionalDocumentStates.AddRange(documents.Select(d => KeyValuePairUtil.Create(d.Id, d))));
         }
 
         public ProjectState AddAnalyzerConfigDocuments(ImmutableArray<AnalyzerConfigDocumentState> documents)

src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs

@@ -776,7 +776,12 @@ public Solution AddAdditionalDocument(DocumentId documentId, string name, Source
 
         public Solution AddAdditionalDocument(DocumentInfo documentInfo)
         {
-            var newState = _state.AddAdditionalDocument(documentInfo);
+            return AddAdditionalDocuments(ImmutableArray.Create(documentInfo));
+        }
+
+        public Solution AddAdditionalDocuments(ImmutableArray<DocumentInfo> documentInfos)
+        {
+            var newState = _state.AddAdditionalDocuments(documentInfos);
             if (newState == _state)
             {
                 return this;

src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs

@@ -1196,27 +1196,11 @@ public SolutionState AddDocuments(ImmutableArray<DocumentInfo> documentInfos)
             return newSolutionState;
         }
 
-        public SolutionState AddAdditionalDocument(DocumentInfo documentInfo)
+        public SolutionState AddAdditionalDocuments(ImmutableArray<DocumentInfo> documentInfos)
         {
-            if (documentInfo == null)
-            {
-                throw new ArgumentNullException(nameof(documentInfo));
-            }
-
-            CheckContainsProject(documentInfo.Id.ProjectId);
-            CheckNotContainsAdditionalDocument(documentInfo.Id);
-
-            var oldProject = this.GetProjectState(documentInfo.Id.ProjectId);
-
-            var state = new TextDocumentState(
-                documentInfo,
-                _solutionServices);
-
-            var newProject = oldProject.AddAdditionalDocument(state);
-            var documentStates = SpecializedCollections.SingletonEnumerable(newProject.GetAdditionalDocumentState(documentInfo.Id));
-
-            return this.ForkProject(newProject,
-                    newFilePathToDocumentIdsMap: CreateFilePathToDocumentIdsMapWithAddedDocuments(documentStates));
+            return AddDocumentsToMultipleProjects(documentInfos,
+                (documentInfo, project) => new TextDocumentState(documentInfo, _solutionServices),
+                (projectState, documents) => (projectState.AddAdditionalDocuments(documents), translationAction: null));
         }
 
         public SolutionState AddAnalyzerConfigDocuments(ImmutableArray<DocumentInfo> documentInfos)