[Bug libgomp/83792] [openacc] Factor out async argument utility functions

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83792

--- Comment #2 from Tom de Vries <vries at gcc dot gnu.org> ---
pinged: https://gcc.gnu.org/ml/gcc-patches/2018-05/msg00046.html