[clang] [llvm] [HLSL] Implement Texture2D::Gather and Texture2D::GatherCmp (PR #183323)

[Helena Kotas via cfe-commits]

hekota wrote:

Can you please remind me why do you need the partial template specialization? It looks to me like the `getGatherReturnType` takes care of creating the correct return type for the `Gather*` methods, even without the partial template.

https://github.com/llvm/llvm-project/pull/183323