glsl/drirc: add an option for gl_ClipVertex / gl_CullDistance checks

glsl/drirc: add an option for gl_ClipVertex / gl_CullDistance checks

The GLSL spec says it's an error if a shader statically writes to these
2 variables.

Until this commit, Mesa refused to link a shader if it had an unused
function writing to one of these variables while another (used) function
wrote to the other.

This commit adds an option to perform dead function elimination after
the intra-stage linking step but before performing these checks.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12897>