fix(llvm): add_files_to_view() got unexpected argument

Hi @emuller, In the last build tests there is this error when creating the env view, from the add_files_to_view() defined in the llvm package:

==> Error: add_files_to_view() got an unexpected keyword argument 'skip_if_exists'

The skip_if_exists keyword has been there for a long time, before Spack v0.18.0, and the method def was fixed in the ebrains repo, until I copied your llvm spack package last week.

getting rid of the error is easy, I just wanted to ask, since this can't have worked (for you) since spack v0.18.0, maybe it's not really needed?