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This change implements the resolution of LWG issue 3320 by removing the
members cbegin(), cend(), crbegin() and crend() from base::span. This
was done because std::cbegin(std::span<T>()) and std::span<T>().cbegin()
returned different results if T was non-const.

Note that the nested const_iterator type is not yet removed from
base::span, as this breaks using it in gMock matchers like ElementsAre.
This will be addressed once gMock no longer requires the presence of
this nested type.

References:
* https://wg21.link/LWG3320
* https://github.com/cplusplus/draft/commit/6faa9c

TBR=dcheng

Bug: 828324
Change-Id: I21f8ffaa0c7183d6e63a087b73f271b677d13f3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086231
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747216}