[OpenMP] Make kernels have protected visibility

This patch changes the kernels generated by OpenMP to have protected
visibility. This is unlikely to change anything functionally. However,
protected visibility better matches the behaviour of these GPU kernels.
We do not expect any pending shared library load to preempt these
kernels so we can specify a more restrictive visibility.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D136198