Thrift: docstring revamp step 1.

Summary:
Allow constants to be documented.
Now any "Definition" can be documented, so we can move the docstring stuff
up to a a higher node in the parse tree.

Reviewed By: mcslee

Test Plan:
Recompiled thrift.
Checked out DocTest.thrift and a few other thrifts.

Revert Plan: ok


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665200 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/compiler/cpp/src/parse/t_const.h b/compiler/cpp/src/parse/t_const.h
index 9cae45b..dadcfd2 100644
--- a/compiler/cpp/src/parse/t_const.h
+++ b/compiler/cpp/src/parse/t_const.h
@@ -18,7 +18,7 @@
  *
  * @author Mark Slee <mcslee@facebook.com>
  */
-class t_const {
+class t_const : public t_doc {
  public:
   t_const(t_type* type, std::string name, t_const_value* value) :
     type_(type),