[IR] Add isAssignable property to IrValueParameter.

Use it to check that only the value parameters that are explicitly
marked assignable are assigned.

Currently, the only parameters marked assignable are those for
default argument stubs.

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

@@ -379,7 +379,7 @@ interface IrBuilderExtension {
         fun irValueParameter(descriptor: ParameterDescriptor): IrValueParameter = with(descriptor) {
             factory.createValueParameter(
                 function.startOffset, function.endOffset, SERIALIZABLE_PLUGIN_ORIGIN, IrValueParameterSymbolImpl(this),
-                name, indexOrMinusOne, type.toIrType(), varargElementType?.toIrType(), isCrossinline, isNoinline
+                name, indexOrMinusOne, type.toIrType(), varargElementType?.toIrType(), isCrossinline, isNoinline, false
             ).also {
                 it.parent = function
             }