JVM_IR: keep the $default suffix for stubs for @JvmName functions

Technically a backwards compatibility problem, as the new backend
*consistently* renamed `f$default` on `f` with `@JvmName("g")` to
`g` instead of `g$default`, so it all worked out. However, this
breaks when encountering libraries compiled with the non-IR backend.

compiler/testData/codegen/box/jvmName/functionWithDefault.kt

@@ -0,0 +1,11 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+@JvmName("bar")
+fun foo(x: String = (object {}).javaClass.enclosingMethod.name) = x
+
+fun box(): String {
+    val f = foo()
+    if (f != "bar\$default") return "Fail: $f"
+    return "OK"
+}