glsl: Move is_builtin flag back to ir_function_signature.

This effectively reverts b6f15869b3.

In desktop GLSL, defining a function with the same name as a built-in
hides that built-in function completely, so there would never be
built-in and user function signatures in the same ir_function.

However, in GLSL ES, overloading built-ins is allowed, and does not
hide the built-in signatures - so we're back to needing this.

src/glsl/linker.cpp

@@ -740,14 +740,14 @@ link_intrastage_shaders(GLcontext *ctx,
 	       ir_function_signature *sig =
 		  (ir_function_signature *) iter.get();
 
-	       if (!sig->is_defined || f->is_builtin)
+	       if (!sig->is_defined || sig->is_builtin)
 		  continue;
 
 	       ir_function_signature *other_sig =
 		  other->exact_matching_signature(& sig->parameters);
 
 	       if ((other_sig != NULL) && other_sig->is_defined
-		   && !other_sig->function()->is_builtin) {
+		   && !other_sig->is_builtin) {
 		  linker_error_printf(prog,
 				      "function `%s' is multiply defined",
 				      f->name);