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/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrGeneratorUtils.kt

@@ -12,7 +12,6 @@ import org.jetbrains.kotlin.ir.expressions.IrBody
 import org.jetbrains.kotlin.ir.expressions.impl.IrDelegatingConstructorCallImpl
 import org.jetbrains.kotlin.ir.expressions.impl.IrInstanceInitializerCallImpl
 import org.jetbrains.kotlin.ir.types.IrSimpleType
-import org.jetbrains.kotlin.ir.util.constructors
 import org.jetbrains.kotlin.ir.util.primaryConstructor
 
 // TODO KT-53096
@@ -38,9 +37,7 @@ fun IrPluginContext.generateBodyForDefaultConstructor(declaration: IrConstructor
     return irFactory.createBlockBody(-1, -1, listOf(delegatingAnyCall, initializerCall))
 }
 
-val Sequence<IrConstructor>.primary get() = find { it.isPrimary } ?: error("Expected to have a primary constructor")
-
-fun IrClass.addDefaultConstructorIfAbsent(ctx: IrPluginContext) {
-    val declaration = constructors.primary
+fun IrClass.addDefaultConstructorBodyIfAbsent(ctx: IrPluginContext) {
+    val declaration = primaryConstructor ?: return
     if (declaration.body == null) declaration.body = ctx.generateBodyForDefaultConstructor(declaration)
 }

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrPredicates.kt

@@ -208,6 +208,9 @@ internal val List<IrConstructorCall>.hasAnySerialAnnotation: Boolean
 internal val List<IrConstructorCall>.serialNameValue: String?
     get() = findAnnotation(SerializationAnnotations.serialNameAnnotationFqName)?.getStringConstArgument(0) // @SerialName("foo")
 
+
+val IrClass.primaryConstructorOrFail get() = primaryConstructor ?: error("$this is expected to have a primary constructor")
+
 /**
  * True — ALWAYS
  * False — NEVER

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializableCompanionIrGenerator.kt

@@ -53,7 +53,7 @@ class SerializableCompanionIrGenerator(
             val serializableClass = getSerializableClassByCompanion(companionDescriptor) ?: return
             if (serializableClass.shouldHaveGeneratedMethodsInCompanion) {
                 SerializableCompanionIrGenerator(irClass, getSerializableClassByCompanion(irClass)!!, context).generate()
-                irClass.addDefaultConstructorIfAbsent(context)
+                irClass.addDefaultConstructorBodyIfAbsent(context)
                 irClass.patchDeclarationParents(irClass.parent)
             }
         }

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt

@@ -199,7 +199,7 @@ open class SerializerIrGenerator(
     fun generateGenericFieldsAndConstructor(typedConstructorDescriptor: IrConstructor) =
         addFunctionBody(typedConstructorDescriptor) { ctor ->
             // generate call to primary ctor to init serialClassDesc and super()
-            val primaryCtor = irClass.constructors.primary
+            val primaryCtor = irClass.primaryConstructorOrFail
             +IrDelegatingConstructorCallImpl.fromSymbolOwner(
                 startOffset,
                 endOffset,
@@ -518,7 +518,7 @@ open class SerializerIrGenerator(
 
             generateGoldenMaskCheck(bitMasks, properties, localSerialDesc.get())
 
-            val ctor: IrConstructorSymbol = serializableIrClass.constructors.primary.symbol
+            val ctor: IrConstructorSymbol = serializableIrClass.primaryConstructorOrFail.symbol
             val params = ctor.owner.valueParameters
 
             val variableByParamReplacer: (ValueParameterDescriptor) -> IrExpression? = { vpd ->
@@ -642,7 +642,7 @@ open class SerializerIrGenerator(
                 // replace origin only for plugin generated serializers
                 irClass.origin = SERIALIZATION_PLUGIN_ORIGIN
             }
-            irClass.addDefaultConstructorIfAbsent(context)
+            irClass.addDefaultConstructorBodyIfAbsent(context)
             irClass.patchDeclarationParents(irClass.parent)
         }
     }