[libomptarget][nfc] Remove data_sharing type aliasing

[libomptarget][nfc] Remove data_sharing type aliasing

Libomptarget previous used __kmpc_data_sharing_slot to access values of type
__kmpc_data_sharing_{worker,master}_slot_static. This aliasing violation was
benign in practice. The master type has since been removed, so a single type
can be used instead.

This is particularly helpful for the transition to an openmp deviceRTL, as the
c++/openmp compiler for amdgcn currently rejects the flexible array member for
being an incomplete type. Serves the same purpose as abandoned D86324.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93075