[clang] [HLSL][NFC] Refactor structured buffer methods tests (PR #161908)

[Helena Kotas via cfe-commits]

hekota wrote:

> Your tests don't make use of the SPV filecheck prefix. Is there output that's only visible when targeting DXIL?

Yes, anytime the codegen emits `@llvm.dx.resource*` intrinsic that is just for DXIL. The SPIR-V tests are not enabled (or implemented yet), that's why there is no SPV prefix use yet. The intention is that this test should eventually be run for both DXIL and SPIRV.

https://github.com/llvm/llvm-project/pull/161908