[OpenMP] Add a semantic check for updating hidden or internal values

A previous patch removed the compiler generating offloading entries
for variables that were declared on the device but were internal or
hidden. This allowed us to compile programs but turns any attempt to run
'#pragma omp target update' on one of those variables a silent failure.
This patch adds a check in the semantic analysis for if the user is
attempting the update a variable on the device from the host that is not
externally visible.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D122403