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.

[email protected]

Bug: 40256175
Change-Id: I38b97f88dc9b092a4fbbe5a3e9e0a54836b6eb8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6513058
Auto-Submit: Jan Keitel <[email protected]>
Reviewed-by: Xiangdong Kong <[email protected]>
Commit-Queue: Xiangdong Kong <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1456425}
NOKEYCHECK=True
GitOrigin-RevId: 3cb388f732344f62593d308457c6b0994571358c
2 files changed