Support full pipeline for basic kotlinx.serialization case:

FIR frontend and IR backend plugins.

Note: IR plugin is now broken for old FE, fixed in subsequent commits

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrPluginContext.kt

@@ -148,6 +148,7 @@ class Fir2IrPluginContext(private val components: Fir2IrComponents) : IrPluginCo
         error(ERROR_MESSAGE)
     }
 
+    @Deprecated("Use classId overload instead")
     override fun referenceConstructors(classFqn: FqName): Collection<IrConstructorSymbol> {
         error(ERROR_MESSAGE)
     }