llvm: replace @when with internal check in @run_before (#8092)
llvm: replace @when with internal check in @run_before (#8092)

Fixes #8088

#7012 added a @when condition for a @run_before check to constrain
that check to only run on Darwin. @when is intended to be used to
choose one of several different implementations of a given function
and cannot be used to conditionally deactivate a check altogether.

This replaces the external decorator with a check that executes at
the beginning of the function.
matthiasdiener/spack,krafczyk/spack,tmerrick1/spack,krafczyk/spack,LLNL/spack,krafczyk/spack,LLNL/spack,mfherbst/spack,iulian787/spack,mfherbst/spack,LLNL/spack,matthiasdiener/spack,iulian787/spack,krafczyk/spack,mfherbst/spack,tmerrick1/spack,tmerrick1/spack,krafczyk/spack,iulian787/spack,tmerrick1/spack,LLNL/spack,LLNL/spack,mfherbst/spack,iulian787/spack,matthiasdiener/spack,matthiasdiener/spack,mfherbst/spack,matthiasdiener/spack,iulian787/spack,tmerrick1/spack