[OpenMP] Added memory scope to atomic::inc API and used the device scope in reduction.

With https://reviews.llvm.org/D137524, memory scope and ordering
attributes are being used to generate the required instructions for
atomic inc/dec on AMDGPU. This patch adds the memory scope attribute to
the atomic::inc API and uses the device scope in reduction. Without
the device scope in atomic_inc, the default system scope leads to
unnecessary L2 write-backs/invalidates.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D154172