JVM IR: introduce ClassGeneratorExtension

This is a low-level extension point for Kotlin/JVM, which is supposed to
be used instead of ClassBuilderInterceptorExtension.

 #KT-54758 Fixed
 #KT-56814 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/state/GenerationState.kt

@@ -397,7 +397,7 @@ class GenerationState private constructor(
                         ?.let { destination -> SignatureDumpingBuilderFactory(it, File(destination)) } ?: it
                 }
             )
-            .wrapWith(ClassBuilderInterceptorExtension.getInstances(project)) { classBuilderFactory, extension ->
+            .wrapWith(loadClassBuilderInterceptors()) { classBuilderFactory, extension ->
                 extension.interceptClassBuilderFactory(classBuilderFactory, originalFrontendBindingContext, diagnostics)
             }
 
@@ -405,6 +405,16 @@ class GenerationState private constructor(
         this.factory = ClassFileFactory(this, interceptedBuilderFactory, finalizers)
     }
 
+    @Suppress("UNCHECKED_CAST")
+    private fun loadClassBuilderInterceptors(): List<ClassBuilderInterceptorExtension> {
+        // Using Class.forName here because we're in the old JVM backend, and we need to load extensions declared in the JVM IR backend.
+        val adapted = Class.forName("org.jetbrains.kotlin.backend.jvm.extensions.ClassBuilderExtensionAdapter")
+            .getDeclaredMethod("getExtensions", Project::class.java)
+            .invoke(null, project) as List<ClassBuilderInterceptorExtension>
+
+        return ClassBuilderInterceptorExtension.getInstances(project) + adapted
+    }
+
     fun beforeCompile() {
         markUsed()
     }