move expensive layout sanity check to debug assertions
It is [hard to fix](https://github.com/rust-lang/rust/issues/141006#issuecomment-2883415000) the slowness in the uninhabitedness computation for very big types but we can fix the very specific case of them being called during the layout sanity checks, as described in #140944.
So this PR moves this uninhabitedness check to the other expensive layout sanity checks that are ran under `debug_assertions`.
It makes building the `lemmy_api_routes` crate's self-profile `layout_of` query go from
```
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
| Item | Self time | % of total time | Time | Item count | Incremental result hashing time |
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
| layout_of | 63.02s | 41.895 | 244.26s | 123703 | 50.30ms |
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
```
on master (2m17s total), to
```
| layout_of | 330.21ms | 0.372 | 26.90s | 123703 | 53.19ms |
```
with this PR (1m15s total).
(Note that the [perf run results](https://github.com/rust-lang/rust/pull/141039#issuecomment-2884688756) below look a bit better than [an earlier run](https://perf.rust-lang.org/compare.html?start=4eca99a18eab3d4e28ed1ce3ee620d442955a470&end=c4a00993f8ee02c7565e7be652608817ea2fb97d&stat=instructions:u) I did in another PR. There may be some positive noise there, or post-merge results could differ a bit)
Since we discussed this today, r? `@compiler-errors` — and cc `@lcnr` and `@RalfJung.`
ac17c3486c