Replace gl_frag_attrib enum with gl_varying_slot.

This patch makes the following search-and-replace changes:

gl_frag_attrib -> gl_varying_slot
FRAG_ATTRIB_* -> VARYING_SLOT_*
FRAG_BIT_* -> VARYING_BIT_*

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Brian Paul <brianp@vmware.com>
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index adcfda8..29856b0 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -1783,7 +1783,7 @@
    if (prog->_LinkedShaders[MESA_SHADER_FRAGMENT] != NULL) {
       link_invalidate_variable_locations(
             prog->_LinkedShaders[MESA_SHADER_FRAGMENT],
-            FRAG_ATTRIB_VAR0, FRAG_RESULT_DATA0);
+            VARYING_SLOT_VAR0, FRAG_RESULT_DATA0);
    }
 
    /* FINISHME: The value of the max_attribute_index parameter is