[FIR] Unwrap dispatcher type when determining property stability

If the modality of a property is not final, the modality of the dispatch
receiver can be used to determine if the property is stable. When
looking up the dispatch receiver symbol, make sure to unwrap the lower
bounds of flexible types.

^KT-61735 Fixed

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/VariableStorageImpl.kt

@@ -25,6 +25,7 @@ import org.jetbrains.kotlin.fir.resolve.toSymbol
 import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.types.ConeClassLikeType
+import org.jetbrains.kotlin.fir.types.lowerBoundIfFlexible
 import org.jetbrains.kotlin.fir.types.resolvedType
 import org.jetbrains.kotlin.fir.utils.exceptions.withFirEntry
 import org.jetbrains.kotlin.fir.utils.exceptions.withFirSymbolEntry
@@ -179,8 +180,8 @@ class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
             property.modality != Modality.FINAL -> {
                 val dispatchReceiver = (originalFir.unwrapElement() as? FirQualifiedAccessExpression)?.dispatchReceiver ?: return null
 
-                val receiverType = (dispatchReceiver.resolvedType as? ConeClassLikeType)?.fullyExpandedType(session) ?: return null
-                val receiverSymbol = receiverType.lookupTag.toSymbol(session) ?: return null
+                val receiverType = dispatchReceiver.resolvedType.lowerBoundIfFlexible().fullyExpandedType(session)
+                val receiverSymbol = (receiverType as? ConeClassLikeType)?.lookupTag?.toSymbol(session) ?: return null
                 when (val receiverFir = receiverSymbol.fir) {
                     is FirAnonymousObject -> PropertyStability.STABLE_VALUE
                     is FirRegularClass -> if (receiverFir.modality == Modality.FINAL) PropertyStability.STABLE_VALUE else PropertyStability.PROPERTY_WITH_GETTER