[Bug analyzer/97116] Fix argument numbering in C++ member function calls

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97116

--- Comment #3 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
is_method seems to be set by:

if (TREE_CODE (TREE_TYPE (fn)) == METHOD_TYPE)

so perhaps we can simply reimplement this logic