Fix several new CA1830 diagnostics (#121131)

From updates in dotnet/sdk#51215

Author: stephentoub

src/coreclr/tools/Common/Compiler/DisplayNameHelpers.cs

@@ -157,7 +157,7 @@ public override Unit AppendName(StringBuilder sb, ArrayType type, FormatOptions
                 AppendName(sb, type.ElementType, options);
                 sb.Append('[');
                 if (type.Rank > 1)
-                    sb.Append(new string(',', type.Rank - 1));
+                    sb.Append(',', type.Rank - 1);
                 sb.Append(']');
                 return default;
             }

src/coreclr/tools/Common/Compiler/ProcessLinkerXmlBase.cs

@@ -579,7 +579,7 @@ public override void AppendName(StringBuilder sb, ArrayType type)
                 AppendName(sb, type.ElementType);
                 sb.Append('[');
                 if (type.Rank > 1)
-                    sb.Append(new string(',', type.Rank - 1));
+                    sb.Append(',', type.Rank - 1);
                 sb.Append(']');
             }
             public override void AppendName(StringBuilder sb, ByRefType type)

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/CustomConverterTests/CustomConverterTests.ReadAhead.cs

@@ -66,7 +66,7 @@ private static string CreateTestStringProperty(int stringSize)
         private static void AppendTestString(int i, int stringSize, StringBuilder builder)
         {
             builder.Append(@"""");
-            builder.Append(new string(i.ToString()[0], stringSize));
+            builder.Append(i.ToString()[0], stringSize);
             builder.Append(@"""");
         }
 

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexNode.cs

@@ -3169,7 +3169,7 @@ public override string ToString()
                     curNode = curNode.Child(curChild);
                     curChild = 0;
 
-                    sb.Append(new string(' ', stack.Count * 2)).Append(curNode.Describe()).AppendLine();
+                    sb.Append(' ', stack.Count * 2).Append(curNode.Describe()).AppendLine();
                 }
                 else
                 {

src/tools/illink/src/linker/Linker/Driver.cs

@@ -185,7 +185,7 @@ public static void ParseResponseFile(TextReader reader, Queue<string> result)
                         numBackslash /= 2;
                     }
                     if (numBackslash > 0)
-                        argBuilder.Append(new string('\\', numBackslash));
+                        argBuilder.Append('\\', numBackslash);
                     if (cur < 0 || (!inquote && char.IsWhiteSpace((char)cur)))
                         break;
                     if (copyChar)