Revert "Fix heavy integer parsing in FunctionN class factory"

This reverts commit 9d493d995e.

core/descriptors/src/org/jetbrains/kotlin/builtins/functions/BuiltInFictitiousFunctionClassFactory.kt

@@ -43,7 +43,12 @@ public class BuiltInFictitiousFunctionClassFactory(
                 val prefix = kind.classNamePrefix
                 if (!className.startsWith(prefix)) continue
 
-                val arity = toInt(className.substring(prefix.length())) ?: continue
+                val arity = try {
+                    className.substring(prefix.length()).toInt()
+                }
+                catch (e: NumberFormatException) {
+                    continue
+                }
 
                 // TODO: validate arity, should be <= 255 for functions, <= 254 for members/extensions
                 return KindWithArity(kind, arity)
@@ -51,16 +56,6 @@ public class BuiltInFictitiousFunctionClassFactory(
 
             return null
         }
-
-        private fun toInt(s: String): Int? {
-            var result = 0
-            for (c in s) {
-                val d = c - '0'
-                if (d !in 0..9) return null
-                result = result * 10 + d
-            }
-            return result
-        }
     }
 
     override fun createClass(classId: ClassId): ClassDescriptor? {