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

[Ross Brunton via llvm-commits]

RossBrunton wrote:

Had another quick look at this and it looks like adding support for host memory not allocated through olMemAlloc is relatively trivial, so I've added it (as well as a helper function).

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