[Bug c++/81676] Wrong warning with unused-but-set-parameter within 'if constexpr'

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81676

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The problem seems to be that we're losing DECL_READ_P on the parm_decl v.