Allow UserSemantic decoration for variables in storage classes other than Input and Output

[AlexeySotkin]

AFAIK the UserSemantics decoration came from graphics domain (SPV_GOOGLE_hlsl_functionality1). Currently the spec says that the variable must be in Input or Output storage classes.
But one might want to use the decoration for OpenCL kernels as well. Is it possible to relax this restriction to allow to apply UserSemantics decoration to variables in other storage classes, i.e. at least in UniformConstant, Workgroup, CrossWorkgroup, Function and Generic ?

[johnkslang]

I don't see any issue generalizing it. The practical issue will be if the tool chain (including validator) knows and agrees will all the uses. So, having the standard specify the set of uses is valuable. We can take this up with Khronos.