Bug#1120402: pocl upgrade to llvm 21 or 19

[Claude Heiland-Allen]

I would like to ask, please use the same llvm version as mesa (currently 
19 in trixie and forky, 21 in sid), because otherwise there are problems 
trying to use both pocl-opencl-icd and mesa-opencl-icd in the same 
program (symptom: segfault crashes with backtraces show version confusion).

I had a look at changing build-deps of trixie's version from 18 to 19, 
but I think it needs upstream changes for changed LLVM apis - my build 
failed.

Thanks,


Claude