The current backend uses direct field access to the backing field
instead of calling the companion object accessor, which calls
an accessibility bridge, which then gets the field for code such as:
```
class A {
companion object {
val s: String = "OK"
}
// f uses direct access to the A.s backing field.
fun f() = s
}
```
This change does the same for the IR backend.
'allopen' compiler plug-in can make data classes and their members open,
which is a compilation error in usual case, but makes sense for Spring
and other frameworks that generate proxy-classes.
Iaroslav Postovalov