[OpenMP] Do not add wrapper headers if using '-nogpuinc'

The OpenMP offloading toolchain uses wrapper headers to implement some
standard features on the GPU. Currently there is no way to turn these
off without also disabling all the standard includes altogether. This
patch makes `-nogpuinc` apply to these wrapper headers so we can use a
sterile toolchain. This was causing problems when attempting to compile
a `libc` for the GPU using OpenMP.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D138598