Bug#1066095: llvm-18 is available in unstable, please also build spirv-llvm-translator-18 in unstable

[Matthias Klose]

spirv-headers are now updated in unstable.

Please could you check if you're able to build with the LLVM-18 in 
unstable? asking because I already updated the b-d in LLVM-18 to 
spirv-llvm-18, which I need to revert, if you cannot build it.