[Bug c++/69523] -Wliteral-suffix should not warn within namespace std

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69523

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Patch submitted to gcc-patches with docs:
https://gcc.gnu.org/ml/gcc-patches/2017-02/msg01109.html