Thrift: docstring revamp step 2.

Thrift: docstring revamp step 2.

Summary:
It was a bad idea to let doxygen comments become a part of the parse tree.
We now get them a totally different way.  The lexer stashes the docsting
contents in a global, and the parser actions (not the rules) pull it out.
This should prevent doxygen comments from ever causing parse errors.

Blame Rev: 52678, 52732

Reviewed By: mcslee

Test Plan:
Recompiled thrift.
Thrifted a bunch of files and saw no parse errors (or C++ compile errors).
Thrifted DocTest.thrift with dump_docs on.

Revert Plan: ok

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665201 13f79535-47bb-0310-9956-ffa450edef68