[llvm] [SPIRV] Add intrinsic to represent spec constants (PR #143179)

[Steven Perron via llvm-commits]

s-perron wrote:

It look like a builtin already exists for this. I should use the one that already exists. I did not see it earlier.

https://github.com/s-perron/llvm-project/blob/fadeeac5a79cbae0edffbabbe383d7451c254afb/llvm/test/CodeGen/SPIRV/transcoding/spec_const.ll

https://github.com/llvm/llvm-project/pull/143179