[llvm] [DirectX] Implement the DXILCBufferAccess pass (PR #134571)

[Chris B via llvm-commits]

https://github.com/llvm-beanz approved this pull request.

This looks reasonable. I think we're going to have to think about how to handle cases where LLVM or Clang might do silly things (like generate memcpy) because there isn't a sane way to represent that memory layout is different in different address spaces.

PR #134174 does cause clang to generate memcpy for array->rvalue casts, but the same will occur for aggregate assignments (see this [C++ example](https://godbolt.org/z/bKdnxe7dh)).

We will either need to teach clang not to generate `memcpy` or we'll need to have some way to legalize it.

https://github.com/llvm/llvm-project/pull/134571