FIR: Refine inference constraints when type variable in flexible position

That issue might be fixed via changing
TypeVariableMarker.shouldBeFlexible at ConeConstraintSystemUtilContext
but this and some other tricks have been added because of incorrect
handling of constraints where type variable has a flexible bound

^KT-51168 Fixed

core/descriptors/src/org/jetbrains/kotlin/types/checker/ClassicTypeSystemContext.kt

@@ -25,6 +25,7 @@ import org.jetbrains.kotlin.resolve.substitutedUnderlyingType
 import org.jetbrains.kotlin.resolve.unsubstitutedUnderlyingType
 import org.jetbrains.kotlin.types.*
 import org.jetbrains.kotlin.types.error.ErrorTypeKind
+import org.jetbrains.kotlin.types.error.ErrorUtils
 import org.jetbrains.kotlin.types.model.*
 import org.jetbrains.kotlin.types.typeUtil.*
 import org.jetbrains.kotlin.utils.addIfNotNull
@@ -33,7 +34,6 @@ import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 import org.jetbrains.kotlin.types.typeUtil.isSignedOrUnsignedNumberType as classicIsSignedOrUnsignedNumberType
 import org.jetbrains.kotlin.types.typeUtil.isStubType as isSimpleTypeStubType
 import org.jetbrains.kotlin.types.typeUtil.isStubTypeForBuilderInference as isSimpleTypeStubTypeForBuilderInference
-import org.jetbrains.kotlin.types.error.ErrorUtils
 import org.jetbrains.kotlin.types.typeUtil.isStubTypeForVariableInSubtyping as isSimpleTypeStubTypeForVariableInSubtyping
 
 interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSystemCommonBackendContext {
@@ -876,12 +876,7 @@ interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSy
         return (baseType.memberScope as? SubstitutingScope)?.substitutor
     }
 
-    override fun SimpleTypeMarker.createConstraintPartForLowerBoundAndFlexibleTypeVariable(): KotlinTypeMarker =
-        if (this.isMarkedNullable()) {
-            this
-        } else {
-            createFlexibleType(this, this.withNullability(true))
-        }
+    override fun useRefinedBoundsForTypeVariableInFlexiblePosition(): Boolean = false
 
     override fun substitutionSupertypePolicy(type: SimpleTypeMarker): TypeCheckerState.SupertypesPolicy {
         require(type is SimpleType, type::errorMessage)