This simplifies several instances of metaprogramming by leveraging
[constraints and concepts from C++20][1]. This:
* gets rid of `std::enable_if` by usage of `requires`, making it more
readable and yield better compiler messages.
* uses `requires` instead of `static_assert` to enforce `TrapLabel`
typing
* simplifies all compile-time tests for validity of a given expression
* uses some standard library concepts where possible
* generalizes and simplifies `SwiftLocationExtractor`
Notice that in order to use the `std::derived_from` concept, `virtual`
inheritance had to be added to the label tags, because diamond
inheritance is a problem otherwise. That's because
`std::derived_from<T, U>` requires that `T*` be convertible to `U*`,
which is false if there are multiple non-virtual inheritance paths from
`U` to `T`. As tags never get actually instantiated, there is no runtime
performance penalty in using `virtual` inheritance.
[1]: https://en.cppreference.com/w/cpp/language/constraints
* visiting now happens in a later stage than fetching labels. While
fetching a list of entities to be visited is created, and then acted
upon in actual extraction. This partially flattens the recursive
nature of `fetchLabel` into a loop inside `SwiftVisitor::extract`.
Recursion in `fetchLabel` will only happen on labels fetched while
naming an entity (calling into `SwiftMangler`).
* The choice whether to name a declaration or type has been moved from
the translators to `SwiftMangler`. Acting on this choice is contained
in `SwiftDispatcher::createLabel`.
* The choice whether to emit a body of a declaration has been moved from
`DeclTranslator` to the dispatcher. This choice is also contained in
`SwiftDispatcher::createLabel`.
* The simple functionality of the `LabelStore` has been moved to the
`SwiftDispatcher` as well.
This is done by adding a `isSuccessfullyExtracted` predicate that is
filled for primary files at the very end of the extractor invocation if
the frontend was performed successfully. If for example the extractor
crashes this will therefore not be filled.
The upgrade script is written so that `SuccessfullyExtractedFiles.ql`
on an upgraded script will give exactly the same results as before it.
