Bug#1068961: llvm-toolchain-18: block migration to testing: to many llvm-toolchain versions in testing

[Drew Parsons]

> If possible, it would be sensible in principle for debian to drop
> llvm-toolchain-15 and llvm-toolchain-16.
> i.e. only keep llvm-toolchain-14 (for llvmlite/numba)
> and the latest llvm-toolchain

It belatedly occurs to me that it could be dangerous to drop the interim 
versions of llvm, since at some point llvmlite will upgrade its llvm 
support, and won't necessarily jump directly to the most recent llvm.

The latest llvmlite (released two weeks ago) now builds against 
llvm-toolchain-15,
https://llvmlite.readthedocs.io/en/latest/release-notes.html

More discussion on the Debian Science mailing list at
https://lists.debian.org/debian-science/2024/06/msg00024.html