[llvm] [SPIR-V] Emit OpConstant instead of OpConstantNull to conform to NonSemantic.Shader.DebugInfo.100 DebugTypeBasic's flags definition (PR #118333)

[Nathan Gauër via llvm-commits]

https://github.com/Keenuts commented:

I'm fine with this PR, but we talked a bit about this issue internally, and we believe this is not a bug in LLVM. It looks like a simple spirv-val oversight.
I'll ping Alan on the verifier bit see if we can move forward with a fix in spirv-val instead.

https://github.com/llvm/llvm-project/pull/118333