glsl: move shader_enums into nir

glsl: move shader_enums into nir

First step towards inverting the dependency between glsl and nir (so nir
can be used without glsl).  Also solves this issue with 'make distclean'

  Making distclean in mesa
  make[2]: Entering directory '/mnt/sdb1/Src64/Mesa-git/mesa/src/mesa'
  Makefile:2486: ../glsl/.deps/shader_enums.Plo: No such file or directory
  make[2]: *** No rule to make target '../glsl/.deps/shader_enums.Plo'. Stop.
  make[2]: Leaving directory '/mnt/sdb1/Src64/Mesa-git/mesa/src/mesa'
  Makefile:684: recipe for target 'distclean-recursive' failed
  make[1]: *** [distclean-recursive] Error 1
  make[1]: Leaving directory '/mnt/sdb1/Src64/Mesa-git/mesa/src'
  Makefile:615: recipe for target 'distclean-recursive' failed
  make: *** [distclean-recursive] Error 1

Reported-by: Andy Furniss <adf.lists@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Rob Clark <robclark@freedesktop.org>