Issue 12967 - Prefix method 'this' qualifiers should be disallowed in DeclDefs scope

https://issues.dlang.org/show_bug.cgi?id=12967

Merge prefix storage classes into `FuncDeclaration::storage_class`.
Ok, but repeating this logic (which is indeed in func.c without explanation) emphasizes that it needs to be encapsulated, not copy/pasted.