[OpenMP] Change default visibility to protected for device declarations

This patch changes the special-case handling of visibility when
compiling for an OpenMP target offloading device. This was orignally
added as a precaution against the bug encountered in PR41826 when
symbols in the device were being preempted by shared library symbols.
This should instead be done by making the visibility protected by default.
With protected visibility we are asserting that the symbols on the device
will never be preempted or preempt another symbol pending a shared library
load.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D117806