[FE] Fix "Not reachable case" exception

Review: https://jetbrains.team/p/kt/reviews/9967

This commit fixes:

    Cause 2: java.lang.IllegalStateException: Not reachable case. We can always suggest making `open val` property `final`
            at org.jetbrains.kotlin.resolve.DeclarationsChecker.reportMustBeInitialized(DeclarationsChecker.kt:845)
            at org.jetbrains.kotlin.resolve.DeclarationsChecker.checkPropertyInitializer(DeclarationsChecker.kt:778)
            at org.jetbrains.kotlin.resolve.DeclarationsChecker.checkProperty(DeclarationsChecker.kt:614)
            at org.jetbrains.kotlin.resolve.DeclarationsChecker.process(DeclarationsChecker.kt:106)
            at org.jetbrains.kotlin.resolve.BodyResolver.resolveBodies(BodyResolver.java:258)

Reproducible in K1 & K2

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

@@ -0,0 +1,11 @@
+// FIR_IDENTICAL
+// LANGUAGE: -ProhibitOpenValDeferredInitialization
+// DIAGNOSTICS: -DEBUG_INFO_LEAKING_THIS
+open class A {
+    <!MUST_BE_INITIALIZED_WARNING!>open val c: Int<!>
+        <!VAL_WITH_SETTER!>set(value) {}<!>
+
+    init {
+        c = 1
+    }
+}