Update UWP package to latest. Change symchk errors to warnings. Chang…

…e ReadOnlySpan indexer to return ref readonly. Related to https://github.com/dotnet/corefx/issues/24105 / dotnet/coreclr#14727.

[tfs-changeset: 1683794]

src/System.Private.CoreLib/shared/System/ReadOnlySpan.cs

@@ -165,15 +165,18 @@ public bool IsEmpty
         /// <exception cref="System.IndexOutOfRangeException">
         /// Thrown when index less than 0 or index greater than or equal to Length
         /// </exception>
-        public T this[int index]
-        {
 #if PROJECTN
+        public ref readonly T this[int index]
+        {
             [BoundsChecking]
             get
             {
-                return Unsafe.Add(ref _pointer.Value, index);
+                return ref Unsafe.Add(ref _pointer.Value, index);
             }
+        }
 #else
+        public T this[int index]
+        {
             [Intrinsic]
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             [NonVersionable]
@@ -183,8 +186,8 @@ public T this[int index]
                     ThrowHelper.ThrowIndexOutOfRangeException();
                 return Unsafe.Add(ref _pointer.Value, index);
             }
-#endif
         }
+#endif
 
         /// <summary>
         /// Copies the contents of this read-only span into destination span. If the source