Add isIdentical to StringGuts and use it for comparation

stdlib/public/core/StringUTF16View.swift

@@ -1064,6 +1064,6 @@ extension String.UTF16View {
   /// - Complexity: O(1)
   @available(SwiftStdlib 6.3, *)
   public func isTriviallyIdentical(to other: Self) -> Bool {
-    self._guts.rawBits == other._guts.rawBits
+    self._guts.isTriviallyIdentical(to: other._guts)
   }
 }