Add an API to enumerate AnalyzerConfigDocuments

We had APIs for add/remove, but didn't actually add the collection
type yet.

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

 *REMOVED*Microsoft.CodeAnalysis.Workspace.ClearOpenDocument(Microsoft.CodeAnalysis.DocumentId documentId, bool isSolutionClosing = false) -> void
+Microsoft.CodeAnalysis.AnalyzerConfigDocument
+Microsoft.CodeAnalysis.Project.AnalyzerConfigDocuments.get -> System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.AnalyzerConfigDocument>
+Microsoft.CodeAnalysis.Project.ContainsAnalyzerConfigDocument(Microsoft.CodeAnalysis.DocumentId documentId) -> bool
+Microsoft.CodeAnalysis.Project.GetAnalyzerConfigDocument(Microsoft.CodeAnalysis.DocumentId documentId) -> Microsoft.CodeAnalysis.AnalyzerConfigDocument
 Microsoft.CodeAnalysis.ProjectInfo.AnalyzerConfigDocuments.get -> System.Collections.Generic.IReadOnlyList<Microsoft.CodeAnalysis.DocumentInfo>
 Microsoft.CodeAnalysis.ProjectInfo.WithAnalyzerConfigDocuments(System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.DocumentInfo> analyzerConfigDocuments) -> Microsoft.CodeAnalysis.ProjectInfo
 Microsoft.CodeAnalysis.Solution.AddAdditionalDocuments(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.DocumentInfo> documentInfos) -> Microsoft.CodeAnalysis.Solution

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

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Microsoft.CodeAnalysis
+{
+    public sealed class AnalyzerConfigDocument : TextDocument
+    {
+        internal AnalyzerConfigDocument(Project project, AnalyzerConfigDocumentState state)
+            : base(project, state)
+        {
+        }
+    }
+}

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

@@ -24,6 +24,7 @@ public partial class Project
         private readonly ProjectState _projectState;
         private ImmutableHashMap<DocumentId, Document> _idToDocumentMap = ImmutableHashMap<DocumentId, Document>.Empty;
         private ImmutableHashMap<DocumentId, TextDocument> _idToAdditionalDocumentMap = ImmutableHashMap<DocumentId, TextDocument>.Empty;
+        private ImmutableHashMap<DocumentId, AnalyzerConfigDocument> _idToAnalyzerConfigDocumentMap = ImmutableHashMap<DocumentId, AnalyzerConfigDocument>.Empty;
 
         internal Project(Solution solution, ProjectState projectState)
         {
@@ -169,6 +170,11 @@ internal Project(Solution solution, ProjectState projectState)
         /// </summary>
         public IEnumerable<TextDocument> AdditionalDocuments => _projectState.AdditionalDocumentIds.Select(GetAdditionalDocument);
 
+        /// <summary>
+        /// All the <see cref="AnalyzerConfigDocument"/>s associated with this project.
+        /// </summary>
+        public IEnumerable<AnalyzerConfigDocument> AnalyzerConfigDocuments => _projectState.AnalyzerConfigDocumentStates.Select(s => GetAnalyzerConfigDocument(s.Key));
+
         /// <summary>
         /// True if the project contains a document with the specified ID.
         /// </summary>
@@ -185,6 +191,14 @@ public bool ContainsAdditionalDocument(DocumentId documentId)
             return _projectState.ContainsAdditionalDocument(documentId);
         }
 
+        /// <summary>
+        /// True if the project contains an <see cref="AnalyzerConfigDocument"/> with the specified ID.
+        /// </summary>
+        public bool ContainsAnalyzerConfigDocument(DocumentId documentId)
+        {
+            return _projectState.ContainsAnalyzerConfigDocument(documentId);
+        }
+
         /// <summary>
         /// Get the documentId in this project with the specified syntax tree.
         /// </summary>
@@ -227,6 +241,19 @@ public TextDocument GetAdditionalDocument(DocumentId documentId)
             return ImmutableHashMapExtensions.GetOrAdd(ref _idToAdditionalDocumentMap, documentId, s_createAdditionalDocumentFunction, this);
         }
 
+        /// <summary>
+        /// Get the analyzer config document in this project with the specified document Id.
+        /// </summary>
+        public AnalyzerConfigDocument GetAnalyzerConfigDocument(DocumentId documentId)
+        {
+            if (!ContainsAnalyzerConfigDocument(documentId))
+            {
+                return null;
+            }
+
+            return ImmutableHashMapExtensions.GetOrAdd(ref _idToAnalyzerConfigDocumentMap, documentId, s_createAnalyzerConfigDocumentFunction, this);
+        }
+
         internal DocumentState GetDocumentState(DocumentId documentId)
         {
             return _projectState.GetDocumentState(documentId);
@@ -266,6 +293,12 @@ private static TextDocument CreateAdditionalDocument(DocumentId documentId, Proj
             return new TextDocument(project, project._projectState.GetAdditionalDocumentState(documentId));
         }
 
+        private static readonly Func<DocumentId, Project, AnalyzerConfigDocument> s_createAnalyzerConfigDocumentFunction = CreateAnalyzerConfigDocument;
+        private static AnalyzerConfigDocument CreateAnalyzerConfigDocument(DocumentId documentId, Project project)
+        {
+            return new AnalyzerConfigDocument(project, project._projectState.GetAnalyzerConfigDocumentState(documentId));
+        }
+
         /// <summary>
         /// Tries to get the cached <see cref="Compilation"/> for this project if it has already been created and is still cached. In almost all
         /// cases you should call <see cref="GetCompilationAsync"/> which will either return the cached <see cref="Compilation"/>