[OpenMP] Mark device RTL variables as hidden

This patch changes the visibility of the `__omp_rtl_debug_kind` variable
to be hidden. These variables are only used by the plugin so they do not
need to be read externally. Previously the default visibility prevented
these variables from being completely eliminated in the module.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D117320