Do not use new stdlib API in reflection implementation

Otherwise this brings incompatibility between kotlin-reflect 1.3.70+ and
kotlin-stdlib 1.3.60-.

This commit reverts the relevant parts of c164745301, 5c89f2fa54 and
896512f7cd.

core/descriptors.runtime/src/org/jetbrains/kotlin/descriptors/runtime/structure/reflectClassUtil.kt

@@ -40,6 +40,18 @@ val Class<*>.primitiveByWrapper: Class<*>?
 val Class<*>.wrapperByPrimitive: Class<*>?
     get() = PRIMITIVE_TO_WRAPPER[this]
 
+private val FUNCTION_CLASSES =
+    listOf(
+        Function0::class.java, Function1::class.java, Function2::class.java, Function3::class.java, Function4::class.java,
+        Function5::class.java, Function6::class.java, Function7::class.java, Function8::class.java, Function9::class.java,
+        Function10::class.java, Function11::class.java, Function12::class.java, Function13::class.java, Function14::class.java,
+        Function15::class.java, Function16::class.java, Function17::class.java, Function18::class.java, Function19::class.java,
+        Function20::class.java, Function21::class.java, Function22::class.java
+    ).mapIndexed { i, clazz -> clazz to i }.toMap()
+
+val Class<*>.functionClassArity: Int?
+    get() = FUNCTION_CLASSES[this]
+
 /**
  * NOTE: does not perform a Java -> Kotlin mapping. If this is not expected, consider using KClassImpl#classId instead
  */