xvalue comment

From-SVN: r186186

gcc/cp/cp-tree.h

@@ -4043,8 +4043,8 @@ enum tag_types {
 enum cp_lvalue_kind_flags {
   clk_none = 0,     /* Things that are not an lvalue.  */
   clk_ordinary = 1, /* An ordinary lvalue.  */
-  clk_rvalueref = 2,/* An rvalue formed using an rvalue reference */
-  clk_class = 4,    /* An rvalue of class-type.  */
+  clk_rvalueref = 2,/* An xvalue (rvalue formed using an rvalue reference) */
+  clk_class = 4,    /* A prvalue of class-type.  */
   clk_bitfield = 8, /* An lvalue for a bit-field.  */
   clk_packed = 16   /* An lvalue for a packed field.  */
 };