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

[rguenth at gcc dot gnu.org]

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

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's probably inconvenient for C indeed (pre-dating C99 inline).  Even GCC has
too many static inlines in headers.

I agree that we can warn for static inline declarations with locations that
are not in any included file (not sure how exactly to verify that).