[HLSL] Add helpers to simplify HLSL resource type declarations. NFC

A few changes to HLSLExternalSemaSource and its BuiltinTypeDeclBuilder
to make defining buffer types less verbose. This will make it a lot
easier to see what the differences between the various buffer types
are once we start introducing more of them.

Pull Request: https://github.com/llvm/llvm-project/pull/73967