Fixtype of member types.
This is a product type introduced by fty::defprod.
This contains information about the members of structure and union types [C:6.7.2.1]. This information consists of a name and a type. We do not capture bit fields (including anonymous ones) and we do not capture static assertions. This information mirrors struct-declon.
We call these `member types' because they are the static counterpart of the member values captured by member-value.