!UPSTREAM: gs: Refer to gl_ClipDistance in clip-distance-in-explicit-too-large.geom.

The GLSL 4.10 rules for redeclaration of built-in interface blocks
(which we've chosen to regard as clarifications of GLSL 1.50) heavily
imply that if a given shader doesn't make use of any members of a
built-in interface block, then that shader's declaration of the
built-in interface block (whether implicit or explicit) has no bearing
on linking.  (For instance, only shaders that use a given built-in
interface block need to have matching declarations of it).

It's not clear from the spec how this should affect other errors that
are discovered at link time.  For example, if a shader redeclares
gl_PerVertex with an invalid array size for gl_ClipDistance, but does
not make use of gl_PerVertex, is that an error?

This patch dodges the issue by modifying
clip-distance-in-explicit-too-large.geom to refer to
gl_in[].gl_ClipDistance.

TODO:
- Test on NVIDIA.