Adding Unsafe API Add nuint that is used by SpanHelpers (#15527)

* Adding Unsafe API Add nuint that is used by SpanHelpers

* Change elementOffset to IntPtr in method signature.

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

Author: ahsonkhan

src/Common/src/CoreLib/Internal/Runtime/CompilerServices/Unsafe.cs

@@ -109,6 +109,22 @@ public static ref T Add<T>(ref T source, int elementOffset)
 #endif
         }
 
+        /// <summary>
+        /// Adds an element offset to the given reference.
+        /// </summary>
+        [Intrinsic]
+        [NonVersionable]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static ref T Add<T>(ref T source, IntPtr elementOffset)
+        {
+#if CORECLR
+            typeof(T).ToString(); // Type token used by the actual method body
+            throw new PlatformNotSupportedException();
+#else
+            return ref AddByteOffset(ref source, (IntPtr)(elementOffset * (nint)SizeOf<T>()));
+#endif
+        }
+
         /// <summary>
         /// Adds an element offset to the given pointer.
         /// </summary>