Fixtype of structure declarations [C:6.7.2.1].
This is a product type introduced by fty::defprod.
These are used inside structure and union specifiers: they do not declare structure types, but rather their members. (A better name for these could be `member declarations', but we follow [C] of course.)
For now we only capture structure declarations that consist of a type specifier sequence and a structure declarator that is an object declarator. We do not capture static assertions. We do not capture bit field sizes.