Merge pull request swiftlang#80934 from etcwilde/ewilde/fix-bootstrap-builds

Fix Bootstrap: UTF8EncoddingError

Author: etcwilde

stdlib/public/core/UTF8EncodingError.swift

@@ -256,6 +256,7 @@ extension UTF8 {
           errors.append(adjustedErr)
         }
       }
+      fatalError()
     }
   }
 }