[HLSL][SPIRV] Add the `vk::constant_id` to clang

[s-perron]

This issue will be considered done when the vk::constant_id attribute has been added to clang. There must be appropriate testing of the attribute for errors, and the codegen for these functions will generate a variable with a decoration indicating the specialization constant id given in the attribute. The initializer will kept in another target attribute. We do not want to leave the attribute in the llvm-ir because the variable does not necessarily take on that value.