[OPENMP] Mark global tors/dtors as used.

If the global variables are marked as declare target and they need
ctors/dtors, these ctors/dtors are emitted and then invoked by the
offloading runtime library. They are not explicitly used in the emitted
code and thus can be optimized out. Patch marks these functions as used,
so the optimizer cannot remove these function during the optimization
phase.

llvm-svn: 331879