Remove ProjectState/SolutionState.AddProjectReference

We already have an API for adding multiple project references; let's
just use that at the public entrypoints than trying to maintain
near-duplicate code.

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

@@ -471,14 +471,6 @@ public static bool IsSameLanguage(ProjectState project1, ProjectState project2)
             return project1.LanguageServices == project2.LanguageServices;
         }
 
-        public ProjectState AddProjectReference(ProjectReference projectReference)
-        {
-            Contract.Requires(!this.ProjectReferences.Contains(projectReference));
-
-            return this.With(
-                projectInfo: this.ProjectInfo.WithProjectReferences(this.ProjectReferences.ToImmutableArray().Add(projectReference)).WithVersion(this.Version.GetNewerVersion()));
-        }
-
         public ProjectState RemoveProjectReference(ProjectReference projectReference)
         {
             Contract.Requires(this.ProjectReferences.Contains(projectReference));

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

@@ -401,7 +401,7 @@ internal Solution WithHasAllInformation(ProjectId projectId, bool hasAllInformat
         /// </summary>
         public Solution AddProjectReference(ProjectId projectId, ProjectReference projectReference)
         {
-            var newState = _state.AddProjectReference(projectId, projectReference);
+            var newState = _state.AddProjectReferences(projectId, SpecializedCollections.SingletonEnumerable(projectReference));
             if (newState == _state)
             {
                 return this;

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

@@ -769,35 +769,6 @@ private static async Task<Compilation> ReplaceSyntaxTreesWithTreesFromNewProject
             return compilation.RemoveAllSyntaxTrees().AddSyntaxTrees(syntaxTrees);
         }
 
-        /// <summary>
-        /// Create a new solution instance with the project specified updated to include
-        /// the specified project reference.
-        /// </summary>
-        public SolutionState AddProjectReference(ProjectId projectId, ProjectReference projectReference)
-        {
-            if (projectId == null)
-            {
-                throw new ArgumentNullException(nameof(projectId));
-            }
-
-            if (projectReference == null)
-            {
-                throw new ArgumentNullException(nameof(projectReference));
-            }
-
-            CheckContainsProject(projectId);
-            CheckContainsProject(projectReference.ProjectId);
-            CheckNotContainsProjectReference(projectId, projectReference);
-            CheckNotContainsTransitiveReference(projectReference.ProjectId, projectId);
-
-            CheckNotSecondSubmissionReference(projectId, projectReference.ProjectId);
-
-            var oldProject = this.GetProjectState(projectId);
-            var newProject = oldProject.AddProjectReference(projectReference);
-
-            return this.ForkProject(newProject, withProjectReferenceChange: true);
-        }
-
         /// <summary>
         /// Create a new solution instance with the project specified updated to include
         /// the specified project references.