nir: add how_declared to nir_variable.data

nir: add how_declared to nir_variable.data

Equivalent to the already existing how_declared at GLSL IR. The only
difference is that we are not adding all the declaration_type
available on GLSL, only the one that we will use on the short term. We
would add more mode if needed on the future.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>