Fixes a build warning.

I added a pragma disables a warning complaining about non-CLS compliant members being abstract, and wants me to mark the type as non-CLS compliant. And it is true that this type cannot be extended by all CLS compliant languages. Having said that, if I marked the type as non-CLS all methods that take it as parameter will now have to be marked CLSCompliant(false), yet consumption of concrete encoders is totally CLS compliant, as it’s mainly to be done by calling helper methods in TextEncoderExtensions class, and so I think the warning is a bit too aggressive.

src/System.Text.Encodings.Web/src/System/Text/Encodings/Web/TextEncoder.cs

@@ -2,16 +2,25 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 namespace System.Text.Encodings.Web
-{
+{    
     public abstract class TextEncoder
     {
+        // The following pragma disables a warning complaining about non-CLS compliant members being abstract, 
+        // and wants me to mark the type as non-CLS compliant. 
+        // It is true that this type cannot be extended by all CLS compliant languages. 
+        // Having said that, if I marked the type as non-CLS all methods that take it as parameter will now have to be marked CLSCompliant(false), 
+        // yet consumption of concrete encoders is totally CLS compliant, 
+        // as it’s mainly to be done by calling helper methods in TextEncoderExtensions class, 
+        // and so I think the warning is a bit too aggressive.  
+        #pragma warning disable 3011
         [CLSCompliant(false)]
         public unsafe abstract bool TryEncodeUnicodeScalar(int unicodeScalar, char* buffer, int bufferLength, out int numberOfCharactersWritten);
 
         // all subclasses have the same implementation of this method.
         // but this cannot be made virtual, because it will cause a virtual call to Encodes, and it destroys perf, i.e. makes common scenario 2x slower 
         [CLSCompliant(false)]
         public unsafe abstract int FindFirstCharacterToEncode(char* text, int textLength);
+        #pragma warning restore
 
         public abstract bool Encodes(int unicodeScalar);