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`.
Repetitive code like this urges me to refactor it. For example, there's `StorageClassDeclaration::stcToChars` to make strings out of stc, and `TypeFunction::TypeFunction` to set the type characteristics. I suspect the code would be better off to create a function that turns `tf->isnothrow` into `STCnothrow`, etc., and then a mask of existing STCxxx could be created from that, and then ANDed with `prefixStc` to test for redundancy, and the result turned into a string with `StorageClassDeclaration::stcToCBuffer`. The result would be reusable, too. What do you think?