[clang] [llvm] [AMDGPU] Add a new builtin type for image descriptor rsrc (PR #160258)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> If it's just a struct, why add a built-in type? I don't see the need here. We have the buffer resource type because we use it as a regular pointer in the IR. For the Clang builtins, they can just use `v8i32`.

No, this should be treated as an opaque type. I do not want vector types escaping into code, eventually these should act like pointers too 

https://github.com/llvm/llvm-project/pull/160258