Visitor code has been split between header and sources to speed up
incremental build. Moreover the code was reorganized using a new `infra`
bazel package (and `visitors` got promoted to a bazel package as well).
This allows to avoid bypassing label type correcness in the extractor,
and allows to independently resolve TBD extractions, as with this
approach TBD nodes do have the correctly typed trap label. The TBD
status is now a predicate on the QL side.
This requires:
* a default visit using the correct type, which is achieved via macro
metaprogramming in `VisitorBase.h`, following the way
`swift::ASTVisitor` is programmed
* a mapping from labels to corresponding binding trap entries. The
functor is defined in `TrapTagTraits.h` and instantiated in generated
`TrapEntries.h`
* Binding trap entries for TBD unknown entities must not have any other
field than the `id` (after all, we are supposed to not extract them
yet). This is why all unextracted fields in `schema.yml` have been
commented out, and will be uncommentend when visitors are added
