@Newk-B: It seems that it is radium developer decision:
https://github.com/kmatheussen/radium/blob/master/bin/packages/README
I think that the solution would be creating new issue in radium git and ask author to update his version of faust.
In the meantime, you can modify PKGBUILD to use older version of llvm - I just checked with llvm11 and it builds successfully.
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