Modernize equality operators

This CL is pure clean-up and contains no functionality changes.
Depending on the files covered in the CL (since the CL was generated
using git cl split), it does a subset of the following:
- Remove unneeded operator!= declarations/definitions since C++20 can
  automatically derive those from operator==.
- Default operator== where this is equivalent to the current behavior.
- Default operator<=> where this is equivalent to the current
  behavior.

This CL was uploaded by git cl split.

R=ellyjones@chromium.org

Bug: 40256175
Change-Id: I0ff24698b3ca9b138f85a15bc8bc2c857cbdfbad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6512963
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Auto-Submit: Jan Keitel <jkeitel@google.com>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1456415}