Minor improvements

1. Making modifier list readonly
2. Using hash set and case insensitive search for illegal headers

Author: Lakshmi Priya Sekar

src/Microsoft.DotNet.CodeFormatting/Rules/HasNoIllegalHeadersFormattingRule.cs

@@ -42,18 +42,18 @@ public async Task<Document> ProcessAsync(Document document, CancellationToken ca
             return document.WithSyntaxRoot(syntaxNode.WithLeadingTrivia(newTrivia));
         }
 
-        public static string[] GetIllegalHeaders()
+        public static HashSet<string> GetIllegalHeaders()
         {
             var filePath = Path.Combine(
                 Path.GetDirectoryName(Uri.UnescapeDataString(new UriBuilder(Assembly.GetExecutingAssembly().CodeBase).Path)),
                 "IllegalHeaders.md");
 
             if (!File.Exists(filePath))
             {
-                return new string[] { };
+                return new HashSet<string>();
             }
 
-            return File.ReadAllLines(filePath).Where(l => !l.StartsWith("##") && !l.Equals("")).ToArray();
+            return new HashSet<string>(File.ReadAllLines(filePath).Where(l => !l.StartsWith("##") && !l.Equals("")), StringComparer.OrdinalIgnoreCase);
         }
     }
 }

src/Microsoft.DotNet.CodeFormatting/Rules/HasPrivateAccessorOnFieldNames.cs

@@ -17,7 +17,7 @@ namespace Microsoft.DotNet.CodeFormatting.Rules
     [RuleOrder(9)]
     internal sealed class HasPrivateAccessorOnFieldNames : IFormattingRule
     {
-        private static string[] AccessorModifiers = {"public", "private", "internal", "protected"};
+        private static readonly string[] AccessorModifiers = {"public", "private", "internal", "protected"};
         public async Task<Document> ProcessAsync(Document document, CancellationToken cancellationToken)
         {
             var syntaxRoot = await document.GetSyntaxRootAsync(cancellationToken) as CSharpSyntaxNode;