Use original instead of lowered function to retrieve toString-impl

in string concatenation lowering. Otherwise, inliner tries to get
body of already lowered function, failing.
 #KT-48284 Fixed

compiler/testData/codegen/box/inlineClasses/inlineToString.kt

@@ -0,0 +1,21 @@
+// TARGET_BACKEND: JVM_IR
+// WITH_RUNTIME
+
+import kotlin.jvm.JvmInline
+
+open class Expando {
+    val expansion: Expansion = Expansion()
+}
+
+@JvmInline
+value class Expansion(val map: MutableMap<String, Any?> = mutableMapOf()) {
+    override inline fun toString(): String = "OK"
+}
+
+data class Foo(val i: Int): Expando() {
+    override fun toString(): String {
+        return "$expansion"
+    }
+}
+
+fun box(): String = Foo(0).toString()