[FIR] Don't report MUST_BE_INITIALIZED on private open properites

^KT-59074 Fixed

This solution is not ideal. Ideally, the allopen compiler plugin
shouldn't report `private` properites as `open` KT-64980, but it will
unpredictably break other things.

compiler/testData/diagnostics/tests/backingField/OpenPrivateValDeferredInit.fir.kt

@@ -0,0 +1,8 @@
+// LANGUAGE:+ProhibitOpenValDeferredInitialization
+open class Foo {
+    <!INCOMPATIBLE_MODIFIERS!>private<!> <!INCOMPATIBLE_MODIFIERS!>open<!> val foo: Int
+
+    init {
+        foo = 1
+    }
+}