linker: Add imported functions to the linked IR

Fixes piglit test glsl-function-chain16 and bugzilla #34203.

NOTE: This is a candidate for stable release branches.

src/glsl/link_functions.cpp

@@ -99,9 +99,15 @@ public:
        * details that may be missing.
        */
       ir_function *f = linked->symbols->get_function(name);
-      if (f == NULL)
+      if (f == NULL) {
 	 f = new(linked) ir_function(name);
 
+	 /* Add the new function to the linked IR.
+	  */
+	 linked->symbols->add_function(f);
+	 linked->ir->push_head(f);
+      }
+
       ir_function_signature *linked_sig =
 	 f->exact_matching_signature(&callee->parameters);
       if (linked_sig == NULL) {