glsl/lower_clip_distance: Update symbol table.

This patch modifies the clip distance lowering pass so that the new
symbol it generates (glClipDistanceMESA) is added to the shader's
symbol table.

This will allow a later patch to modify the linker so that it finds
transform feedback varyings using the symbol table rather than having
to iterate through all the declarations in the shader.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index 29fc5d8..802323e 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -2568,8 +2568,9 @@
       if (!prog->LinkStatus)
 	 goto done;
 
-      if (ctx->ShaderCompilerOptions[i].LowerClipDistance)
-         lower_clip_distance(prog->_LinkedShaders[i]->ir);
+      if (ctx->ShaderCompilerOptions[i].LowerClipDistance) {
+         lower_clip_distance(prog->_LinkedShaders[i]);
+      }
 
       unsigned max_unroll = ctx->ShaderCompilerOptions[i].MaxUnrollIterations;