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