(IN-94056) pragma skip_variants XXXX does not remove XXXX_KEYWORD_DECLARED keyword

HI. I found that #pragma skip_variants XXXX does not remove XXXX_KEYWORD_DECLARED keyword.

It simply makes shader compilation error on code like this:

#pragma multi_compile_fog
#pragma skip_variants FOG_LINEAR
#pragma skip_variants FOG_EXP
#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
// Using URP's fog function
... = ComputeFogFactor(...);

Reported as IN-94056.