glsl: Export the compiler's GS layout qualifiers to the gl_shader.

glsl: Export the compiler's GS layout qualifiers to the gl_shader.

Next step is to validate them at link time.

v2 (Paul Berry <stereotype441@gmail.com>): Don't attempt to export the
layout qualifiers in the event of a compile error, since some of them
are set up by ast_to_hir(), and ast_to_hir() isn't guaranteed to have
run in the event of a compile error.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
v3 (Paul Berry <stereotype441@gmail.com>): Use PRIM_UNKNOWN to
represent "not set in this shader".

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>