[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 approved this pull request.

Looks like this might have been an oversight, but several extension specs are using the same requirement, so making a change in the validator might not be trivial.

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