glsl: convert to nir at compile time and drop the glsl ir linker

This is the final MR after years of work by many people to get us to this point. This series moves glsl_to_nir() to compile time and replaces the remaining bits of the GLSL IR linker with a fully nir based linker.

The result here is a cleaner code-base that current mesa devs can better work with, support into the future and better understand. It also sets us up for other possible improvements such as improving the glsl shader cache to finally support storing compiled shaders and not just linked shaders.