[OpenMP][libomptarget] Enable usage of shared memory slots

Summary:
Allow the runtime to use the existing shared memory statically allocated slots.

When a variable is globalized, the underlying memory can be either shared or global memory (both have block-wide visibility). In this case, we allow that the storage to use a limited amount of shared memory that has been statically allocated already. Only if shared memory doesn't prove to be enough do we then invoke malloc() to create a new global memory slot.

Reviewers: ABataev, carlo.bertolli, grokos, caomhin

Reviewed By: grokos

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D44486

llvm-svn: 327639