Hi!

While rebuilding the python related packages against the Python 3.15rc2
version we found that llvm-toolchain-18 fails to build from source [1].
It actually fails to build in general, regardless of Python 3.15, so
setting the severity to serious.

To fix the issues I had to apply a number of upstream patches to handle
kernel changes, cmake changes, gcc16 changes, python 3.14 and 3.15
changes.

On top of this, I also had to update some packaging tooling and
configuration. I'm attaching the debdiff including all the necessary
changes to get the package to build successfully.

I've applied the upstream fix in the sandbox [2] to verify that it
builds successfully, please apply the patch to get llvm-toolchain-18
back to build from source.

Happy hacking,

[1]: https://debusine.debian.net/debian/r-python-python3.15/artifact/4372219/
[2]: https://debusine.debian.net/debian/r-python-python3.15/