PSI2IR KT-41284 use getters for open data class property values

'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.

compiler/testData/ir/irText/classes/openDataClass.kt

@@ -0,0 +1,7 @@
+// This test emulates 'allopen' compiler plugin.
+
+@Suppress("INCOMPATIBLE_MODIFIERS")
+open data class ValidatedProperties(
+    open val test1: String,
+    open val test2: String
+)