Fixtype of specifiers and qualifiers [C:6.7.2.1] [C:A.2.2].
This is a tagged union type, introduced by fty::deftagsum.
This does not correspond directly to any nonterminal in the grammar in [C], but it is useful to define specifier-qualifier-list: see spec/qual-list.
As a GCC extension, we include attribute specifiers. See our ABNF grammar.