Comment diagnostics: warn on duplicate \brief and \return commands.

Doxygen manual claims that multiple \brief or \returns commands will be merged
together, but actual behavior is different (second \brief command becomes a
part of a discussion, second \returns becomes a "Returns: blah" paragraph on
its own).  Anyway, it seems to be a bad idea to use multiple \brief or \returns
commands in a single command.

llvm-svn: 161325