[Bug c++/87407] Enhance -Wunused-function to handle also inline functions

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87407

--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
Like these addressed here:
https://gcc.gnu.org/ml/gcc-patches/2018-09/msg01317.html