Use existing shared helper methods

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers/MaintainabilityRules/SA1402FileMayOnlyContainASingleType.cs

@@ -108,7 +108,7 @@ private static bool ContainsTopLevelTypeDeclarations(SyntaxNode node)
             return node.IsKind(SyntaxKind.CompilationUnit) || node.IsKind(SyntaxKind.NamespaceDeclaration) || node.IsKind(SyntaxKindEx.FileScopedNamespaceDeclaration);
         }
 
-        private static bool IsRelevantType(SyntaxNode node, StyleCopSettings settings)
+        private static bool IsRelevantType(MemberDeclarationSyntax node, StyleCopSettings settings)
         {
             if (IsFileLocalType(node))
             {
@@ -142,18 +142,9 @@ private static bool IsRelevantType(SyntaxNode node, StyleCopSettings settings)
             return isRelevant;
         }
 
-        private static bool IsFileLocalType(SyntaxNode node)
+        private static bool IsFileLocalType(MemberDeclarationSyntax node)
         {
-            const SyntaxKind FileKeyword = (SyntaxKind)8449;
-
-            var modifiers = node switch
-            {
-                BaseTypeDeclarationSyntax x => x.Modifiers,
-                DelegateDeclarationSyntax x => x.Modifiers,
-                _ => default,
-            };
-
-            return modifiers.Any(FileKeyword);
+            return node.GetModifiers().Any(SyntaxKindEx.FileKeyword);
         }
     }
 }