Fix heavy integer parsing in FunctionN class factory

#KT-7924 Fixed

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

@@ -43,12 +43,7 @@ public class BuiltInFictitiousFunctionClassFactory(
                 val prefix = kind.classNamePrefix
                 if (!className.startsWith(prefix)) continue
 
-                val arity = try {
+                val arity = toInt(className.substring(prefix.length())) ?: continue
-                    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)
@@ -56,6 +51,16 @@ 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? {