[FIR] add support for varargs in delegated constructor calls

Also add support for spread operators as named arguments.

compiler/testData/codegen/box/innerNested/superConstructorCall/objectExtendsClassVararg.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 open class SomeClass(val some: Double, val other: Int, vararg val args: String) {
     fun result() = args[1]
 }

compiler/testData/codegen/box/mixedNamedPosition/varargs.kt

@@ -1,5 +1,4 @@
 // !LANGUAGE: +NewInference +MixedNamedArgumentsInTheirOwnPosition
-// IGNORE_BACKEND_FIR: JVM_IR
 
 fun foo1(
     vararg p1: Int,

compiler/testData/codegen/box/privateConstructors/withVarargs.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // See also KT-6299
 public open class Outer private constructor(val s: String, vararg i: Int) {
     class Inner: Outer("xyz")

compiler/testData/codegen/box/secondaryConstructors/varargs.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun join(x: Array<out String>): String {
     var result = ""
     for (i in x) {

compiler/testData/codegen/box/vararg/doNotCopyImmediatelyCreatedArrays.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun booleanVararg(vararg xs: Boolean) {
     if (xs.size != 1 && xs[0] != true) throw AssertionError()
 }

compiler/testData/codegen/box/vararg/varargInFunParam.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box(): String {
     if (test1() != "") return "fail 1"
     if (test1(1) != "1") return "fail 2"