[OpenMP] Mark -fopenmp-implicit-rpath as NoArgumentUnused

This matches the behavior for all the other -fopenmp options,
as well as -frtlib-add-rpath.

For context, Fedora passes this flag by default in case OpenMP is
used, and this results in a warning if it (usually) isn't, which
causes build failures for some programs with unnecessarily strict
build systems (like Ruby).

Differential Revision: https://reviews.llvm.org/D133316