Small C++ PATCH for debug_tree(cp_expr)

[Jason Merrill]

David's cp_expr change made use of the 'pt' gdb macro less convenient in 
some cases, so I'm adding a debug_tree overload to handle it.
commit 48d6e68c6b43b11d4f0bd696f19b490358954937
Author: Jason Merrill <ja...@redhat.com>
Date:   Mon Feb 8 23:46:14 2016 -0500
    	* ptree.c (debug_tree): Implement for cp_expr.

diff --git a/gcc/cp/ptree.c b/gcc/cp/ptree.c
index 2c8ff99..8266e83 100644
--- a/gcc/cp/ptree.c
+++ b/gcc/cp/ptree.c
@@ -287,3 +287,11 @@ cxx_print_xnode (FILE *file, tree node, int indent)
       break;
     }
 }
+
+/* Print the node NODE on standard error, for debugging.  */
+
+DEBUG_FUNCTION void
+debug_tree (cp_expr node)
+{
+  debug_tree (node.get_value());
+}