[libcxx] [llvm] [libc++] Implement P3379R0 Constrain `std::expected` equality operators (PR #135759)

[Hristo Hristov via llvm-commits]

H-G-Hristov wrote:

@yronglin Please add 
```
Closes #118135
```
 to the top comment in the PR to link this PR to the GitHub issue as @frederick-vs-ja suggested above.

https://github.com/llvm/llvm-project/pull/135759