[OpenMP] Ensure memory fences are created with barriers for AMDGPUs

It turns out that the `__builtin_amdgcn_s_barrier()` alone does not emit
a fence. We somehow got away with this and assumed it would work as it
(hopefully) is correct on the NVIDIA path where we just emit a
`__syncthreads`. After talking to @arsenm we now (mostly) align with the
OpenCL barrier implementation [1] and emit explicit fences for AMDGPUs.

It seems this was the underlying cause for #59759, but I am not 100%
certain. There is a chance this simply hides the problem.

Fixes: https://github.com/llvm/llvm-project/issues/59759

[1] https://github.com/RadeonOpenCompute/ROCm-Device-Libs/blob/07b347366eb2c6ebc3414af323c623cbbbafc854/opencl/src/workgroup/wgbarrier.cl#L21

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D145290