[FIR] Implement canBeNull for captured type by checking its supertypes

#KT-62959

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrImplicitCastInserter.kt

@@ -256,6 +256,9 @@ class Fir2IrImplicitCastInserter(
     }
 
     private fun ConeKotlinType.acceptsNullValues(): Boolean {
+        if (this is ConeCapturedType && this.constructor.projection.kind == ProjectionKind.IN) {
+            return constructor.projection.type!!.canBeNull(session)
+        }
         return canBeNull(session) || hasEnhancedNullability
     }
 

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/TypeUtils.kt

@@ -821,6 +821,7 @@ fun ConeKotlinType.canBeNull(session: FirSession): Boolean {
             it.coneType.fullyExpandedType(session).canBeNull(session)
         }
         is ConeIntersectionType -> intersectedTypes.all { it.canBeNull(session) }
+        is ConeCapturedType -> constructor.supertypes?.all { it.canBeNull(session) } == true
         else -> fullyExpandedType(session).isNullable
     }
 }