[OpenMP] Remove hidden visibility for declare target variables

This patch changes the visiblity of variables declared within a declare
target directive. Variable declarations within a declare target
directive need to be externally visible to the plugin for initialization
or reading. Previously this would cause runtime errors where the named
global could not be found because it was not included in the symbol
table.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D117362