glsl/gs: handle gl_ClipDistance geometry input in lower_clip_distance.

glsl/gs: handle gl_ClipDistance geometry input in lower_clip_distance.

This corresponds to the lowering of gl_ClipDistance to
gl_ClipDistanceMESA for vertex and geometry shader outputs.  Since
this lowering pass occurs after lower_named_interface blocks, it deals
with 2D arrays (gl_ClipDistance[vertex][clip_plane]) rather than 1D
arrays in an interface block
(gl_in[vertex].gl_ClipDistance[clip_plane]).

v2 (Paul Berry <stereotype441@gmail.com>): Fix indexing order for
gl_ClipDistance input lowering.  Properly lower bulk assignment of
gl_ClipDistance inputs.  Rework for GLSL 1.50 style geometry shaders.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
v3 (Paul Berry <stereotype441@gmail.com>): Add comments and assertions
to clarify that the 2D version of clip distance is only used for
geometry shader inputs.

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