IR API: change val ... : MutableList to var ...: List for most lists

All mutable state for IR declarations should be either:
- var (mutable properties)
- or class member list

Mutable properties are straightforward to persist.

The class member list is handled in a special way.

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

@@ -44,7 +44,7 @@ interface IrBuilderExtension {
     private fun IrClass.declareSimpleFunctionWithExternalOverrides(descriptor: FunctionDescriptor): IrSimpleFunction {
         return compilerContext.symbolTable.declareSimpleFunction(startOffset, endOffset, SERIALIZABLE_PLUGIN_ORIGIN, descriptor)
             .also { f ->
-                descriptor.overriddenDescriptors.mapTo(f.overriddenSymbols) {
+                f.overriddenSymbols = descriptor.overriddenDescriptors.map {
                     compilerContext.symbolTable.referenceSimpleFunction(it.original)
                 }
             }
@@ -357,16 +357,15 @@ interface IrBuilderExtension {
 
         if (!overwriteValueParameters)
             assert(valueParameters.isEmpty())
-        else
-            valueParameters.clear()
-        valueParameters.addAll(descriptor.valueParameters.map { it.irValueParameter() })
+
+        valueParameters = descriptor.valueParameters.map { it.irValueParameter() }
 
         assert(typeParameters.isEmpty())
         if (copyTypeParameters) copyTypeParamsFromDescriptor()
     }
 
     fun IrFunction.copyTypeParamsFromDescriptor() {
-        descriptor.typeParameters.mapTo(typeParameters) {
+        typeParameters += descriptor.typeParameters.map {
             IrTypeParameterImpl(
                 startOffset, endOffset,
                 SERIALIZABLE_PLUGIN_ORIGIN,

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

@@ -76,7 +76,7 @@ class SerializableCompanionIrGenerator(
             )
         }
 
-        irSerializableClass.annotations.add(annotationCtorCall)
+        irSerializableClass.annotations += annotationCtorCall
     }
 
     override fun generateSerializerGetter(methodDescriptor: FunctionDescriptor) {