glsl: Make gl_ClipDistance[] implicitly sized.

glsl: Make gl_ClipDistance[] implicitly sized.

From the GLSL 1.30 spec, section 7.1 (Vertex Shader Special Variables):

  The gl_ClipDistance array is predeclared as unsized and must be
  sized by the shader either redeclaring it with a size or indexing it
  only with integral constant expressions.

Fixes piglit tests clip-distance-implicit-length.vert,
clip-distance-implicit-nonconst-access.vert, and
{vs,fs}-clip-distance-explicitly-sized.shader_test.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>