Fix compilation error when @Serializable expect class has explicit companion

as it may have no primary constructor (only actual class has it).

Refactor & rename Fir2IrGeneratorUtils.kt as having 'Fir' in stacktrace
when actual K2 compiler is not used is very confusing.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/2134
#KT-55683 Fixed

plugins/kotlinx-serialization/testData/boxIr/expectActual.kt

@@ -0,0 +1,35 @@
+// IGNORE_BACKEND_K2: JVM_IR
+// TARGET_BACKEND: JVM_IR
+// !LANGUAGE: +MultiPlatformProjects
+// WITH_STDLIB
+
+// FILE: common.kt
+
+package foo
+
+import kotlinx.serialization.*
+
+@Serializable
+expect class Expected {
+    companion object { fun factoryMethod(): Expected }
+}
+
+// FILE: jvm.kt
+
+package foo
+
+import kotlinx.serialization.*
+
+@Serializable
+actual class Expected {
+    actual companion object { actual fun factoryMethod(): Expected = Expected() }
+}
+
+
+fun box(): String {
+    val b = Expected.factoryMethod()
+    if (b !is Expected) return "Incorrect factory method"
+    val desc = Expected.serializer().descriptor
+    if (desc.toString() != "foo.Expected()") return "Incorrect descriptor $desc"
+    return "OK"
+}