There's a merged pull request on the void-packages repository

rust: update to 1.39.0, cargo: update to 0.40.0
https://github.com/void-linux/void-packages/pull/16243

Description:
This also switches to LLVM9.

~~Generally seems to work fine except Firefox fails to build now. Fix is probably to be done in Firefox. Some generated code from `style` fails to compile, with errors like~~

```
19:20.28 error[E0119]: conflicting implementations of trait `std::clone::Clone` for type `gecko_bindings::structs::root::mozilla::GeckoBackground`:
```

~~As the generated file is one line, it spams the console really badly,~~ but turns out that is not a new issue, so we can ~~probably~~ ignore that, ~~as long as FF can get fixed~~.

Unbreak status:

- [x] `firefox`
- [x] `firefox-esr`
- [x] `thunderbird`
- [x] `icecat`