mesa: set up gl_vert_result and gl_frag_attrib values for gl_ClipDistance.

This patch assigns enumerated values for gl_ClipDistance in the
gl_vert_result and gl_frag_attrib enums, so that driver back-ends can
assign gl_ClipDistance to the appropriate hardware registers.  It also
adjusts the functions _mesa_vert_result_to_frag_attrib() and
_mesa_frag_attrib_to_vert_result() (which translate between the two
enums) to correctly translate the new enumerated values.

Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Brian Paul <brianp@vmware.com>
diff --git a/src/glsl/ir_variable.cpp b/src/glsl/ir_variable.cpp
index e0b6f38..58be64b 100644
--- a/src/glsl/ir_variable.cpp
+++ b/src/glsl/ir_variable.cpp
@@ -621,9 +621,9 @@
    const glsl_type *const clip_distance_array_type =
       glsl_type::get_array_instance(glsl_type::float_type, 0);
 
-   /* FINISHME: gl_ClipDistance needs a real location assigned. */
    add_variable(instructions, state->symbols,
-		"gl_ClipDistance", clip_distance_array_type, ir_var_out, -1);
+		"gl_ClipDistance", clip_distance_array_type, ir_var_out,
+                VERT_RESULT_CLIP_DIST0);
 
 }
 
@@ -841,9 +841,9 @@
    const glsl_type *const clip_distance_array_type =
       glsl_type::get_array_instance(glsl_type::float_type, 0);
 
-   /* FINISHME: gl_ClipDistance needs a real location assigned. */
    add_variable(instructions, state->symbols,
-		"gl_ClipDistance", clip_distance_array_type, ir_var_in, -1);
+		"gl_ClipDistance", clip_distance_array_type, ir_var_in,
+                FRAG_ATTRIB_CLIP_DIST0);
 }
 
 static void