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.
Alexander Udalov
a7e48c3af1