FIR mangler: support definitely not-null types (for Java)

compiler/fir/fir2ir/jvm-backend/src/org/jetbrains/kotlin/fir/backend/jvm/FirJvmMangleComputer.kt

@@ -234,6 +234,10 @@ open class FirJvmMangleComputer(
                     tBuilder.appendSignature(suffix)
                 }
             }
+            is ConeDefinitelyNotNullType -> {
+                // E.g. not-null type parameter in Java
+                mangleType(tBuilder, type.original)
+            }
             else -> error("Unexpected type $type")
         }
     }