[Wasm] Simple support for external classes and interfaces

* Map these types to Wasm externref
* Don't generate any declaration code for now

Casting and accessing members is not supported yet

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/DeclarationGenerator.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.descriptors.DescriptorVisibilities
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.ir.IrBuiltIns
 import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.backend.js.utils.isJsExport
 import org.jetbrains.kotlin.ir.backend.js.utils.findUnitGetInstanceFunction
 import org.jetbrains.kotlin.ir.backend.js.utils.realOverrideTarget
 import org.jetbrains.kotlin.ir.declarations.*
@@ -173,6 +174,7 @@ class DeclarationGenerator(val context: WasmModuleCodegenContext) : IrElementVis
 
     override fun visitClass(declaration: IrClass) {
         if (declaration.isAnnotationClass) return
+        if (declaration.isExternal) return
         val symbol = declaration.symbol
 
 

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/TypeTransformer.kt

@@ -90,7 +90,7 @@ class WasmTypeTransformer(
                 val klass = this.getClass()
                 val ic = context.backendContext.inlineClassesUtils.getInlinedClass(this)
 
-                if (klass != null && klass.hasWasmForeignAnnotation()) {
+                if (klass != null && (klass.hasWasmForeignAnnotation() || klass.isExternal)) {
                     WasmExternRef
                 } else if (ic != null) {
                     context.backendContext.inlineClassesUtils.getInlineClassUnderlyingType(ic).toWasmValueType()