Reduce allocation in TextWriter.NewLine (#121508)

It's reasonably common for a codebase to pick a specific newline string
and set it regardless of OS, but currently that ends up allocating on
every OS each call to set_NewLine.

Two changes:
1. When setting TextWriter.NewLine to the existing value, make it a nop
to avoid unnecessarily calling ToCharArray.
2. When setting TextWriter.NewLine to "\n" on Windows or to "\r\n" on
Unix, use a cached array.

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System/IO/TextWriter.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Buffers;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Runtime.CompilerServices;
@@ -25,6 +26,9 @@ public abstract partial class TextWriter : MarshalByRefObject, IDisposable, IAsy
         // We don't want to allocate on every TextWriter creation, so cache the char array.
         private static readonly char[] s_coreNewLine = Environment.NewLineConst.ToCharArray();
 
+        // s_coreNewLine will be ['\r', '\n'] on Windows and ['\n'] on Unix. This is the opposite, lazily created/cached.
+        private static char[]? s_otherCoreNewLine;
+
         /// <summary>
         /// This is the 'NewLine' property expressed as a char[].
         /// It is exposed to subclasses as a protected field for read-only
@@ -112,8 +116,17 @@ public virtual string NewLine
             {
                 value ??= Environment.NewLineConst;
 
-                CoreNewLineStr = value;
-                CoreNewLine = value.ToCharArray();
+                if (CoreNewLineStr != value)
+                {
+                    CoreNewLineStr = value;
+                    CoreNewLine =
+                        value == Environment.NewLineConst ? s_coreNewLine : // current OS default
+                        Environment.NewLineConst == "\r\n" && value == "\n" ? (s_otherCoreNewLine ??= ['\n']) : // other OS default
+                        Environment.NewLineConst == "\n" && value == "\r\n" ? (s_otherCoreNewLine ??= ['\r', '\n']) : // other OS default
+                        value.ToCharArray(); // unknown
+                }
+
+                Debug.Assert(CoreNewLineStr == new string(CoreNewLine));
             }
         }