[Bug c++/69375] GCC allows PMF type "void (T::*)()" to be caught as "void (T::*)() const"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69375

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
My front-end debugging skills are pitiful, but I've found something suspicious.
ptm_initializer uses TYPE_PTRMEM_POINTED_TO_TYPE to get that pointee type. For
this case that expands to TYPE_PTRMEMFUNC_FN_TYPE which is a call to
cp_build_qualified_type with the qualifiers from cp_type_quals.

But cp_type_quals tries pretty hard to ensure we never get cv-quals for a
function type. For the purposes of RTTI, where we really do care about the
difference between void() and void()const, do we want the memfn quals instead?