[clang] [llvm] [clang][amdgpu] Add builtins for raw/struct buffer lds load (PR #137678)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> There's some existing builtin stuff going on in this area, e.g. [96e94b5](https://github.com/llvm/llvm-project/commit/96e94b5662c613fd80f712080751076254a73524)

That's different, the direct buffer usage is still useful, probably.

> I'm leaning slightly towards an automatic conversion between the two forms within LLVM not being worth the hassle. Inclined to abandon this PR, looks like it doesn't make sense within the wider context, but will leave it open for a while in case people comment further.

It does, we should have a consistent set of buffer builtins 

https://github.com/llvm/llvm-project/pull/137678