For `__has_result_type`, it can be replaced with a variable template
`__has_result_type_v`. Note that the pre-existing extraneous `*` used in
detection is buggy, but it's a functional change to fix it.
`false_type` and `true_type` are no longer directly used, so direct
inclusion of `<__type_traits/integral_constant.h>` is removed.
For `__derives_from_{unary,binary}_function`, it's unnecessary to invent
a `__two` type for each specialization. So `void` is used instead. Also,
`nullptr` is now used instead of `0`.
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