KT-63269: implementation for SIRGenerator with top-level functions

KT-63269: add test generator

Merge-request: KT-MR-12945
Merged-by: Artem Olkov <artem.olkov@jetbrains.com>

native/swift/sir/src/org/jetbrains/kotlin/sir/SirElement.kt

@@ -9,4 +9,13 @@ package org.jetbrains.kotlin.sir
  * A marker interface that denotes Swift IR elements.
  */
 interface SirElement {
-}
+}
+
+class SirModule : SirElement {
+    val declarations: MutableList<SirDeclaration> =
+        mutableListOf()
+}
+
+interface SirDeclaration
+
+class SirForeignFunction(val fqName: List<String>) : SirDeclaration