[llvm] [Offload] Add `olMemcpyRect` with amdgpu implementation (PR #160321)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@jhuber6 Looks like it still runs the TearDown, which fails because the buffers weren't allocated. Should be fixed now.

https://github.com/llvm/llvm-project/pull/160321