Improve toString of platform types created by typeOf

In the stdlib implementation, render "!" if the type is only
nullability-flexible. Otherwise, render "($lower..$upper)".

Note that full kotlin-reflect has a much more complicated logic (see
`DescriptorRendererImpl.renderFlexibleType`) that renders things like
`(Mutable)List` and so on. It is not a goal of the stdlib implementation
to replicate all of that, since it requires copying a large amount of
code, namely the entirety of `JavaToKotlinClassMap` to map Java class
names to Kotlin.

libraries/stdlib/jvm/runtime/kotlin/jvm/internal/TypeReference.kt

@@ -53,7 +53,22 @@ public class TypeReference /* @SinceKotlin("1.6") constructor */(
             else arguments.joinToString(", ", "<", ">") { it.asString() }
         val nullable = if (isMarkedNullable) "?" else ""
 
-        return klass + args + nullable
+        val result = klass + args + nullable
+
+        return when (val upper = platformTypeUpperBound) {
+            is TypeReference -> {
+                when (val renderedUpper = upper.asString(true)) {
+                    result -> {
+                        // Rendered upper bound can be the same as rendered lower bound in cases when the type is mutability-flexible,
+                        // but not nullability-flexible, since both MutableCollection and Collection are rendered as "java.util.Collection".
+                        result
+                    }
+                    "$result?" -> "$result!"
+                    else -> "($result..$renderedUpper)"
+                }
+            }
+            else -> result
+        }
     }
 
     private val Class<*>.arrayClassName