Rollup merge of #146117 - GuillaumeGomez:fix-search-index-generation, r=notriddle

Fix search index generation

Fixes this issue:

```
error: couldn't generate documentation: failed to read column from disk: data consumer error: missing field `unknown number` at line 1 column 8
  |
  = note: failed to create or modify "build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/search.index/entry/": failed to read column from disk: data consumer error: missing field `unknown number` at line 1 column 8

warning: `theme_css` (lib doc) generated 1 warning
error: could not document `theme_css`
```

The problem was that a conversion was forgotten for the `ItemType` enum.

Thanks a lot to `@janis-bhm!`

r? `@lolbinarycat`

Author: GuillaumeGomez