Fix DeclarePublicAPIAnalyzer

The analyzer can report three different diagnostics, but currently only
returns two from the `SupportedDiagnostics` property.

src/Diagnostics/Roslyn/Core/ApiDesign/DeclarePublicAPIAnalyzer.cs

@@ -89,7 +89,7 @@ public class DeclarePublicAPIAnalyzer : DiagnosticAnalyzer
             MethodKind.EventRemove
         };
 
-        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(DeclareNewApiRule, RemoveDeletedApiRule);
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(DeclareNewApiRule, RemoveDeletedApiRule, ExposedNoninstantiableType);
 
         public override void Initialize(AnalysisContext context)
         {