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`.
For example, with `@safe @system void foo() {}`, `prefixStc` will be set to `STCsafe | STCtrusted`. But the conflicting has been already reported as an error in `parseDeclDefs` function.

And, this section is consistent with the code in `FuncDeclaration::semantic`, `TypeFuntion::TypeFunction`, and `TypeFunction::semantic`. I know there's implicit attribute priority order: safe < system < trusted. I'm just following it.