[ping] Fix ICE in pp_cxx_unqualified_id (PR c++/88348)

[Zhouyi Zhou]

Hello,

Ping for https://gcc.gnu.org/ml/gcc-patches/2018-12/msg00151.html

By the way, are there any GCC open project for volunteer beginners to

do in spare time.

Thanks in advance for your feedback,

Zhouyi