[JS IR] Introduce jsArguments() intrinsic function

`jsArguments()` is lowered into a reference of the `arguments` object.

This is useful for extracting varargs, when calling Kotlin code from
JavaScript. For a concrete example, see KT-15223.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsIntrinsics.kt

@@ -215,6 +215,8 @@ class JsIntrinsics(private val irBuiltIns: IrBuiltIns, val context: JsIrBackendC
 
     val unreachable = getInternalFunction("unreachable")
 
+    val jsArguments = getInternalFunction("jsArguments")
+
     val returnIfSuspended = getInternalFunction("returnIfSuspended")
     val getContinuation = getInternalFunction("getContinuation")
 
@@ -348,4 +350,4 @@ class JsIntrinsics(private val irBuiltIns: IrBuiltIns, val context: JsIrBackendC
 
     private fun getInternalClassWithoutPackage(fqName: String) =
         context.symbolTable.referenceClass(context.getClass(FqName(fqName)))
-}
+}

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/JsIntrinsicTransformers.kt

@@ -236,6 +236,8 @@ class JsIntrinsicTransformers(backendContext: JsIrBackendContext) {
             add(intrinsics.jsInvokeSuspendSuperType, suspendInvokeTransform)
             add(intrinsics.jsInvokeSuspendSuperTypeWithReceiver, suspendInvokeTransform)
             add(intrinsics.jsInvokeSuspendSuperTypeWithReceiverAndParam, suspendInvokeTransform)
+
+            add(intrinsics.jsArguments) { _, _ -> Namer.ARGUMENTS }
         }
     }
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/utils/Namer.kt

@@ -20,6 +20,8 @@ object Namer {
 
     val IMPLICIT_RECEIVER_NAME = "this"
 
+    val ARGUMENTS = JsNameRef("arguments")
+
     val PROTOTYPE_NAME = "prototype"
     val CONSTRUCTOR_NAME = "constructor"
 
@@ -42,4 +44,4 @@ object Namer {
     const val KCALLABLE_ARITY = "\$arity"
 
     const val SHARED_BOX_V = "_v"
-}
+}

libraries/stdlib/js-ir/runtime/jsIntrinsics.kt

@@ -195,6 +195,9 @@ internal fun <A> slice(a: A): A
 @JsIntrinsic
 internal fun unreachable(): Nothing
 
+@JsIntrinsic
+internal fun jsArguments(): Any?
+
 @JsIntrinsic
 @Suppress("REIFIED_TYPE_PARAMETER_NO_INLINE") // TODO: mark `inline` and skip in inliner
 internal fun <reified T : Any> jsClassIntrinsic(): JsClass<T>
@@ -204,4 +207,4 @@ internal fun <reified T : Any> jsClassIntrinsic(): JsClass<T>
 internal fun jsInIntrinsic(lhs: Any?, rhs: Any): Boolean
 
 @JsIntrinsic
-internal fun jsDelete(e: Any?)
+internal fun jsDelete(e: Any?)