glsl: Make a function to express a GLSL version ir human-readable form.

glsl: Make a function to express a GLSL version ir human-readable form.

This will be useful in generating more helpful error messages,
especially with the addition of GLSL 3.00 ES support.

[v2, idr]: Rename ctx parameter to mem_ctx

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Carl Worth <cworth@cworth.org>