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`.
I just realized that there already was code to convert `tf->isnothrow` into `STCnothrow` in func.c. That doubly convinces me this needs refactoring - we are writing the same code over and over again.