This operation decorate with an Id instead of a Literal or String.

It is used by HlslCounterBufferGOOGLE (provided by
SPV_GOOGLE_hlsl_functionality1).  Even if we don't do anything with
that decoration, we must be able to parse SPIR-V that uses it.

Fixes: 891886da

 "spirv: Add no-op support for VK_GOOGLE_hlsl_functionality1"
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>