nir: Add a descriptor_set field to nir_variable

This is needed for supporting the Vulkan binding model

Reviewed-by: Rob Clark <robdclark@gmail.com>

src/compiler/nir/nir.h

@@ -265,6 +265,11 @@ typedef struct nir_variable {
        */
       int index;
 
+      /**
+       * Descriptor set binding for sampler or UBO.
+       */
+      int descriptor_set;
+
       /**
        * Initial binding point for a sampler or UBO.
        *