Bug#893405: castxml: Please use llvm-defaults (or at least llvm-toolchain-6.0)

[Sylvestre Ledru]

Source: castxml

Hello,

In order to limit the number of versions in llvm, it would be great
if you could move to llvm-toolchain-6.0.
Even better, please try to move to llvm-defaults.

Thanks,
Sylvestre