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
Our mangler is split in two version:
* `SwiftTrapMangler`, with the same behaviour as the previous
`SwiftMangler`, constructing mangled names with trap label references
* `SwiftRecursiveMangler` that replaces trap label references with
recursive calls to its own `mangle` functions, effectively rolling out
the entire chain of references
The latter is used to create lazy trap file names. Hashing is used to
avoid excessively long filenames.
* 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.
In case the extractor is run in isolation for debugging/testing, this
will avoid littering the current working directory with artifacts, and
instead having a single `extractor-out` directory to inspect or clean
up.
Also extractor logs have been nested into a `swift` directory, as the
log directory provided by the `codeql` cli is actually shared between
languages.
