Use original descriptors for functions when generating stubs

Descriptors for builtin class members on JVM are substituted.

compiler/testData/ir/irText/stubs/builtinMap.kt

@@ -0,0 +1,6 @@
+// !DUMP_DEPENDENCIES
+// WITH_RUNTIME
+// FULL_JDK
+
+fun <K, V> Map<out K, V>.plus(pair: Pair<K, V>): Map<K, V> =
+        if (this.isEmpty()) mapOf(pair) else LinkedHashMap(this).apply { put(pair.first, pair.second) }