[OpenCL] Set MinVersion for sub_group_barrier with memory_scope

The memory_scope enum is not available before OpenCL 2.0, so ensure
the sub_group_barrier overload with a memory_scope argument is
restricted to OpenCL 2.0 and above.  This is already the case in
opencl-c.h.

Fixes the issue revealed by https://reviews.llvm.org/D120254

Reported-by: Harald van Dijk (hvdijk)