Add a method to `IDiagnosticAnalyzerService`

Add the `GetDiagnosticDescriptors(DiagnosticAnalyzer)` method to the
`IDiagnosticAnalyzerService` interface.

src/Features/Core/Diagnostics/IDiagnosticAnalyzerService.cs

@@ -70,5 +70,11 @@ internal interface IDiagnosticAnalyzerService
         /// </summary>
         /// <returns>A mapping from analyzer name to the diagnostics produced by that analyzer</returns>
         ImmutableDictionary<string, ImmutableArray<DiagnosticDescriptor>> GetDiagnosticDescriptors(Project projectOpt);
+
+        /// <summary>
+        /// Gets a list of the diagnostics provided by the given <see cref="DiagnosticAnalyzer"/>.
+        /// </summary>
+        /// <returns>A list of the diagnostics produced by the given analyzer</returns>
+        ImmutableArray<DiagnosticDescriptor> GetDiagnosticDescriptors(DiagnosticAnalyzer analyzer);
     }
 }

src/VisualStudio/Core/SolutionExplorerShim/DiagnosticItem/DiagnosticItemProvider.cs

@@ -23,7 +23,7 @@ protected override IAttachedCollectionSource CreateCollectionSource(AnalyzerItem
         {
             if (relationshipName == KnownRelationships.Contains)
             {
-                return new DiagnosticItemSource(item, _commandHandler, (DiagnosticAnalyzerService)_diagnosticAnalyzerService);
+                return new DiagnosticItemSource(item, _commandHandler, _diagnosticAnalyzerService);
             }
 
             return null;

src/VisualStudio/Core/SolutionExplorerShim/DiagnosticItem/DiagnosticItemSource.cs

@@ -20,14 +20,14 @@ internal partial class DiagnosticItemSource : IAttachedCollectionSource
 
         private readonly AnalyzerItem _item;
         private readonly IAnalyzersCommandHandler _commandHandler;
-        private readonly DiagnosticAnalyzerService _diagnosticAnalyzerService;
+        private readonly IDiagnosticAnalyzerService _diagnosticAnalyzerService;
         private BulkObservableCollection<DiagnosticItem> _diagnosticItems;
         private Workspace _workspace;
         private ProjectId _projectId;
         private ReportDiagnostic _generalDiagnosticOption;
         private ImmutableDictionary<string, ReportDiagnostic> _specificDiagnosticOptions;
 
-        public DiagnosticItemSource(AnalyzerItem item, IAnalyzersCommandHandler commandHandler, DiagnosticAnalyzerService diagnosticAnalyzerService)
+        public DiagnosticItemSource(AnalyzerItem item, IAnalyzersCommandHandler commandHandler, IDiagnosticAnalyzerService diagnosticAnalyzerService)
         {
             _item = item;
             _commandHandler = commandHandler;

src/VisualStudio/Core/Test/Diagnostics/ExternalDiagnosticUpdateSourceTests.vb

@@ -185,6 +185,10 @@ Namespace Microsoft.VisualStudio.LanguageServices.UnitTests.Diagnostics
             Public Function GetProjectDiagnosticsForIdsAsync(solution As Solution, Optional projectId As ProjectId = Nothing, Optional diagnosticIds As ImmutableHashSet(Of String) = Nothing, Optional cancellationToken As CancellationToken = Nothing) As Task(Of ImmutableArray(Of DiagnosticData)) Implements IDiagnosticAnalyzerService.GetProjectDiagnosticsForIdsAsync
                 Return SpecializedTasks.EmptyImmutableArray(Of DiagnosticData)()
             End Function
+
+            Public Function GetDiagnosticDescriptors(analyzer As DiagnosticAnalyzer) As ImmutableArray(Of DiagnosticDescriptor) Implements IDiagnosticAnalyzerService.GetDiagnosticDescriptors
+                Return ImmutableArray(Of DiagnosticDescriptor).Empty
+            End Function
         End Class
     End Class
 End Namespace