Auto merge of rust-lang#131045 - compiler-errors:remove-unnamed_field…

…s, r=wesleywiser

Retire the `unnamed_fields` feature for now

`#![feature(unnamed_fields)]` was implemented in part in rust-lang#115131 and rust-lang#115367, however work on that feature has (afaict) stalled and in the mean time there have been some concerns raised (e.g.[^1][^2]) about whether `unnamed_fields` is worthwhile to have in the language, especially in its current desugaring. Because it represents a compiler implementation burden including a new kind of anonymous ADT and additional complication to field selection, and is quite prone to bugs today, I'm choosing to remove the feature.

However, since I'm not one to really write a bunch of words, I'm specifically *not* going to de-RFC this feature. This PR essentially *rolls back* the state of this feature to "RFC accepted but not yet implemented"; however if anyone wants to formally unapprove the RFC from the t-lang side, then please be my guest. I'm just not totally willing to summarize the various language-facing reasons for why this feature is or is not worthwhile, since I'm coming from the compiler side mostly.

Fixes rust-lang#117942
Fixes rust-lang#121161
Fixes rust-lang#121263
Fixes rust-lang#121299
Fixes rust-lang#121722
Fixes rust-lang#121799
Fixes rust-lang#126969
Fixes rust-lang#131041

Tracking:
* rust-lang#49804

[^1]: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Unnamed.20struct.2Funion.20fields
[^2]: rust-lang#49804 (comment)