For Rust's LLVM fork, sure. But as Steve Klabnik noted in the first comment I linked, unmodified LLVM is supported.

In addition, later down in the comment chain:

cycloptic:

> Can't there be a build option to not use the LLVM submodule, and instead use the system LLVM?

steveklabnik:

> There is. We even test that it builds as part of our CI, to make sure it works, IIRC.

For a more concrete example, Fedora supports choosing between system and bundled LLVM when building Rust [0, 1].

[0]: https://news.ycombinator.com/item?id=26222190

[1]: https://src.fedoraproject.org/rpms/rust//blob/rawhide/f/rust...