Migrate boxInline tests to new multi-file framework

compiler/testData/codegen/boxInline/private/privateClass.kt

@@ -0,0 +1,35 @@
+// FILE: 1.kt
+
+package test
+
+private class S {
+    fun a(): String {
+        return "K"
+    }
+}
+
+// This function exposes S which is a private class (package-private in the byte code)
+// It can be accessed outside the `test` package, which may lead to IllegalAccessError.
+// This behavior may be changed later
+internal inline fun call(s: () -> String): String {
+    val s = test()
+    return s() + test2(s)
+}
+
+private fun test(): S {
+    return S()
+}
+
+private fun test2(s: S): String {
+    return s.a()
+}
+
+// FILE: 2.kt
+
+import test.*
+
+fun box(): String {
+    return call {
+        "O"
+    }
+}