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/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java

@@ -57,6 +57,12 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
             runTest("plugins/kotlinx-serialization/testData/boxIr/enumsAreCached.kt");
         }
 
+        @Test
+        @TestMetadata("expectActual.kt")
+        public void testExpectActual() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/expectActual.kt");
+        }
+
         @Test
         @TestMetadata("externalSerialierJava.kt")
         public void testExternalSerialierJava() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java

@@ -55,6 +55,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/enumsAreCached.kt");
     }
 
+    @Test
+    @TestMetadata("expectActual.kt")
+    public void testExpectActual() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/expectActual.kt");
+    }
+
     @Test
     @TestMetadata("externalSerialierJava.kt")
     public void testExternalSerialierJava() throws Exception {