* 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.
Firstly, this change reworks how inter-process races are resolved.
Moreover some responsability reorganization has led to merging
`TrapArena` and `TrapOutput` again into a `TrapDomain` class.
A `TargetFile` class is introduced, that is successfully created
only for the first process that starts processing a given trap output
file. From then on `TargetFile` simply wraps around `<<` stream
operations, dumping them to a temporary file. When `TargetFile::commit`
is called, the temporary file is moved on to the actual target trap
file.
Processes that lose the race can now just ignore the unneeded
extraction and go on, while previously all processes would carry out
all extractions overwriting each other at the end.
Some of the file system logic contained in `SwiftExtractor.cpp` has been
moved to this class, and two TODOs are solved:
* introducing a better inter process file collision avoidance strategy
* better error handling for trap output operations: if unable to write
to the trap file (or carry out other basic file operations), we just
abort.
The changes to `ExprVisitor` and `StmtVisitor` are due to wanting to
hide the raw `TrapDomain::createLabel` from them, and bring more
funcionality under the generic caching/dispatching mechanism.
