Convert another std::result_of to std::invoke_result
Convert another std::result_of to std::invoke_result

While at it, also change result_of_apply to look like
std::invoke_result.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200923013300.810737-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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