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`.
Yes, there is many duplicated code for function type in parse.c, mtype.c, func.c. 
I'd refactor code more clean, but I want a little more time in order to avoid breaking existing behavior, because they are slightly different each other.