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This change implements comparisons of base::spans with different types.
This is useful to compare spans that only differ in their cv qualifiers,
or whose different types allow comparison (e.g. int and double or
std::string and base::StringPiece).

Comparisons across types is a recent addition to the std::span proposal
and was not part of P0122R6.

Bug: 828324
Change-Id: Ia65a027c6b4a141f1a6dad11ba1d7699c7ed6f8c
Reviewed-on: https://chromium-review.googlesource.com/1006580Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550172}