[clang] [Clang][AMDGPU] Add builtins for instrinsic `llvm.amdgcn.raw.ptr.buffer.load` (PR #99258)

[Shilei Tian via cfe-commits]

shiltian wrote:

> probably should change the stores to unsigned to match in a separate patch

Doing it right now.

https://github.com/llvm/llvm-project/pull/99258