rustc: 1.41.0 -> 1.42.0

The patch is included in rust master[1], but neither that, nor the
QuiltOS version we were using previously, apply to 1.42.0, so I've
included the fixed version here.

[1]: https://github.com/rust-lang/rust/commit/4f15867faf2797257cbeb9e4a38ae8dc87dcf2e9