This allows containers that either optimize based on these
concepts or produce lifetime warnings based on them to
handle absl::Span appropriately. An example is Chromium's
base::span, which warns more aggressively about construction
from rvalues that are not borrowed ranges.

This only has an effect for codebases using C++20. While
many such codebases will presumably also be using std::span
directly, they may use absl::Span for backwards compat, or
compile against libraries that do so.

Also fixes lint's that fired when I tried to edit this.

PiperOrigin-RevId: 688552975
Change-Id: I603e04cd74d60ac6b65754ac73037d7f0ab457fe