MUST_BE_INITIALIZED: take into account containingDeclaration's modality

^KT-58587 Fixed
Review: https://jetbrains.team/p/kt/reviews/10136

This commit is important in scope of KT-57553. It makes the migration
more smooth.

Other related tests:
- testUninitializedOrReassignedVariables
- testAugmentedAssignmentInInitializer

compiler/testData/diagnostics/tests/backingField/MustBeInitializedEffectivelyFinalOff.kt

@@ -0,0 +1,12 @@
+// !LANGUAGE:-TakeIntoAccountEffectivelyFinalInMustBeInitializedCheck
+open class Base {
+    open var x: String = ""
+}
+
+class Foo : Base() {
+    <!MUST_BE_INITIALIZED_OR_BE_ABSTRACT!>override var x: String<!>
+
+    init {
+        x = ""
+    }
+}