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This change improves util::ranges::begin() and end() by adding constexpr
support and making them prefer a member function over a free standing
function. This serves to close the gap to std::ranges::begin() and
std::ranges::end() as specified by the C++20 standard.

Furthermore, special constexpr support is added for std::array, which
otherwise only has constexpr support in C++17.

Lastly, it moves the functions from the iterator.h to the ranges.h
header, since in C++20 these functions are present in <ranges>, not
<iterator>.

Bug: 1071094
Change-Id: Id5f3b7ff83b3fb336286f4ed2d4fe8a6cfcf0b9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2325171Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793034}