[Clang][OpenMP] Allow unified_shared_memory for Pascal-generation GPUs.
The Pascal architecture supports the page migration engine required for
unified_shared_memory, as indicated by NVIDIA:
The limitation was introduced in D54493 which justified the cut-off by
the requirement for unified addressing. However, Unified Virtual
Addressing (UVA) is already available with sm20 (Fermi, Kepler,
Unified shared memory might even be possible with these, but with
migration of entire allocations on kernel startup.
Reviewed By: JonChesterfield
Differential Revision: https://reviews.llvm.org/D101595