Proposal: allow std::not_fn, use it instead of base::not_fn

[Danil Chapovalov]

Just to reduce redundancy.

e.g. https://chromium-review.googlesource.com/c/chromium/src/+/3819264

(side note, one place std::not_fn already sneaked in:

https://source.chromium.org/chromium/chromium/src/+/main:components/omnibox/browser/autocomplete_result.cc;l=1073;drc=3afd1937f74d79119c10a141047db51d5414c9a5 )

[Danil Chapovalov]

Sorry, was too hasty: 

std::not_fn is not constexpr until c++20, base::not_fn is constexpr,

and there is code that rely on it to be constexpr.

so it might be better to keep using base::not_fn for now.