[llvm] [HLSL] Allow completely unused cbuffers (PR #164557)

[Justin Bogner via llvm-commits]

bogner wrote:

> Do you have an HLSL sample that produces this? I want to make sure I have a test for SPIR-V, but I'm not sure how to reproduce it.

I hit this with a shader who's entry point wasn't "main", but I hadn't specified a `-E` argument  to the compiler - ie, https://hlsl.godbolt.org/z/7xx6qrYsT

https://github.com/llvm/llvm-project/pull/164557