mesa: Fix transform feedback of gl_ClipDistance.

mesa: Fix transform feedback of gl_ClipDistance.

On drivers that set gl_shader_compiler_options::LowerClipDistance (for
example i965), references to gl_ClipDistance (a float[8] array) will
be converted to references to gl_ClipDistanceMESA (a vec4[2] array).

This patch modifies the linker so that requests for transform feedback
of gl_ClipDistance are similarly converted.

Fixes Piglit test "EXT_transform_feedback/builtin-varyings
gl_ClipDistance".

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>