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)
     }

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/extensions/IrPluginContext.kt

@@ -54,6 +54,7 @@ interface IrPluginContext : IrGeneratorContext {
     @Deprecated("Use classId overload instead")
     fun referenceClass(fqName: FqName): IrClassSymbol?
     fun referenceTypeAlias(fqName: FqName): IrTypeAliasSymbol?
+    @Deprecated("Use classId overload instead")
     fun referenceConstructors(classFqn: FqName): Collection<IrConstructorSymbol>
     @Deprecated("Use callableId overload instead")
     fun referenceFunctions(fqName: FqName): Collection<IrSimpleFunctionSymbol>

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/extensions/IrPluginContextImpl.kt

@@ -121,6 +121,7 @@ open class IrPluginContextImpl constructor(
         }
     }
 
+    @Deprecated("Use classId overload instead")
     override fun referenceConstructors(classFqn: FqName): Collection<IrConstructorSymbol> {
         @Suppress("DEPRECATION")
         val classSymbol = referenceClass(classFqn) ?: error("Cannot find class $classFqn")
@@ -157,6 +158,7 @@ open class IrPluginContextImpl constructor(
     }
 
     override fun referenceConstructors(classId: ClassId): Collection<IrConstructorSymbol> {
+        @Suppress("DEPRECATION")
         return referenceConstructors(classId.asSingleFqName())
     }