libgccjit: Fix infinite recursion in gt_ggc_mx_lang_tree_node

This issue happened when compiling rustc with rustc_codegen_gcc, more
specifically when compiling its rustc_parse crate.

gcc/jit/
	PR target/105827
	* dummy-frontend.cc: Fix lang_tree_node.
	* jit-common.h: New function (jit_tree_chain_next) used by
	lang_tree_node.

gcc/jit/dummy-frontend.cc

@@ -972,12 +972,13 @@ struct GTY(()) lang_identifier
 
 /* The resulting tree type.  */
 
+/* See lang_tree_node in gcc/c/c-decl.cc. */
 union GTY((desc ("TREE_CODE (&%h.generic) == IDENTIFIER_NODE"),
-	   chain_next ("CODE_CONTAINS_STRUCT (TREE_CODE (&%h.generic), TS_COMMON) ? ((union lang_tree_node *) TREE_CHAIN (&%h.generic)) : NULL")))
-lang_tree_node
+	chain_next ("(union lang_tree_node *) jit_tree_chain_next (&%h.generic)"))) lang_tree_node
 {
   union tree_node GTY((tag ("0"),
-		       desc ("tree_node_structure (&%h)"))) generic;
+		       desc ("tree_node_structure (&%h)")))
+	generic;
   struct lang_identifier GTY((tag ("1"))) identifier;
 };
 

gcc/jit/jit-common.h

@@ -93,6 +93,21 @@ const int NUM_GCC_JIT_TYPES = GCC_JIT_TYPE_FLOAT128 + 1;
 
    End of comment for inclusion in the docs.  */
 
+/* See c_tree_chain_next in gcc/c-family/c-common.h.  */
+static inline tree
+jit_tree_chain_next (tree t)
+{
+  /* TREE_CHAIN of a type is TYPE_STUB_DECL, which is different
+     kind of object, never a long chain of nodes.  Prefer
+     TYPE_NEXT_VARIANT for types.  */
+  if (CODE_CONTAINS_STRUCT (TREE_CODE (t), TS_TYPE_COMMON))
+    return TYPE_NEXT_VARIANT (t);
+  /* Otherwise, if there is TREE_CHAIN, return it.  */
+  if (CODE_CONTAINS_STRUCT (TREE_CODE (t), TS_COMMON))
+    return TREE_CHAIN (t);
+  return NULL;
+}
+
 namespace gcc {
 
 namespace jit {