[IR] Make isExternal mutable and accessible via common interface

JS IR: Mark all effectively external declarations as external to access it
directly instead of computing it every time

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/MoveBodilessDeclarationsToSeparatePlace.kt

@@ -48,6 +48,7 @@ private fun isBuiltInClass(declaration: IrDeclaration): Boolean =
 fun moveBodilessDeclarationsToSeparatePlace(context: JsIrBackendContext, moduleFragment: IrModuleFragment) {
     MoveBodilessDeclarationsToSeparatePlaceLowering(context).let { moveBodiless ->
         moduleFragment.files.forEach {
+            markExternalDeclarations(it)
             moveBodiless.lower(it)
         }
     }
@@ -112,3 +113,31 @@ class MoveBodilessDeclarationsToSeparatePlaceLowering(private val context: JsIrB
         }, null)
     }
 }
+
+fun markExternalDeclarations(packageFragment: IrPackageFragment) {
+    for (declaration in packageFragment.declarations) {
+        if (declaration is IrPossiblyExternalDeclaration) {
+            if (declaration.isExternal) {
+                markNestedExternalDeclarations(declaration)
+            }
+        }
+    }
+}
+
+
+fun markNestedExternalDeclarations(declaration: IrDeclaration) {
+    if (declaration is IrPossiblyExternalDeclaration) {
+        declaration.isExternal = true
+    }
+    if (declaration is IrProperty) {
+        declaration.getter?.isExternal = true
+        declaration.setter?.isExternal = true
+        declaration.backingField?.isExternal = true
+    }
+    if (declaration is IrClass) {
+        declaration.declarations.forEach {
+            markNestedExternalDeclarations(it)
+        }
+    }
+}
+