Moves ByReference to shared CoreLib (dotnet/coreclr#21633)

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

netcore/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems

@@ -111,6 +111,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Parser\Utf8Parser.TimeSpan.LittleG.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Parser\Utf8Parser.TimeSpanSplitter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Byte.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\ByReference.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Char.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\CharEnumerator.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\CLSCompliantAttribute.cs" />

netcore/System.Private.CoreLib/shared/System/ByReference.cs

@@ -0,0 +1,42 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Runtime.CompilerServices;
+using System.Runtime.Versioning;
+
+namespace System
+{
+    // ByReference<T> is meant to be used to represent "ref T" fields. It is working
+    // around lack of first class support for byref fields in C# and IL. The JIT and 
+    // type loader has special handling for it that turns it into a thin wrapper around ref T.
+    [NonVersionable]
+    internal readonly ref struct ByReference<T>
+    {
+        // CS0169: The private field '{blah}' is never used
+#pragma warning disable 169
+         private readonly IntPtr _value;
+#pragma warning restore
+
+        [Intrinsic]
+        public ByReference(ref T value)
+        {
+            // Implemented as a JIT intrinsic - This default implementation is for 
+            // completeness and to provide a concrete error if called via reflection
+            // or if intrinsic is missed.
+            throw new PlatformNotSupportedException();
+        }
+
+        public ref T Value
+        {
+            [Intrinsic]
+            get
+            {
+                // Implemented as a JIT intrinsic - This default implementation is for 
+                // completeness and to provide a concrete error if called via reflection
+                // or if the intrinsic is missed.
+                throw new PlatformNotSupportedException();
+            }
+        }
+    }
+}