Don't mangle synthesized 'box' function for inline classes

compiler/testData/codegen/bytecodeText/inlineClasses/synthesizedBoxMethodIsNotMangled.kt

@@ -0,0 +1,11 @@
+// !LANGUAGE: +InlineClasses
+
+inline class Z1(val x: Int)
+inline class Z2(val x: Z1)
+
+fun test(zs: MutableList<Z2>, z: Z2) {
+    zs.add(z)
+}
+
+// 1 public final static box\(I\)LZ2;
+// 1 INVOKESTATIC Z2\$Erased.box \(I\)LZ2;