Use the Josh [1] utility to add `compiler-builtins` as a subtree, which
will allow us to stop using crates.io for updates. This is intended to
help resolve some problems when unstable features change and require
code changes in `compiler-builtins`, which sometimes gets trapped in a
bootstrap cycle.
This was done using `josh-filter` built from the r24.10.04 tag:
git fetch https://github.com/rust-lang/compiler-builtins.git 233434412fe7eced8f1ddbfeddabef1d55e493bd
josh-filter ":prefix=library/compiler-builtins" FETCH_HEAD
git merge --allow-unrelated FILTERED_HEAD
The HEAD in the `compiler-builtins` repository is 233434412f ("fix an if
statement that can be collapsed").
[1]: https://github.com/josh-project/josh
fcb3000340