RFR: Filter unsupported constructs in a separate pass

[Maurizio Cimadamore]

This PR moves all the logic to filter unsupported constructs in a dedicated visitor pass, hence removing all the checks from `OutputFactory`. This makes `OutputFactory` a lot simpler, as now this class only has to check for the presence of the `Skip` attribute.

One map in output factory (to keep track of generated structs) has been removed; we can in fact just set the `Skip` attribute on a scoped declaration as soon as we process it, to achieve the same effect. This map was also used in `visitTypedef`, so that we could fetch the correct supertype for the typedef declaration. This logic has now been replaced by making the `JavaName` attribute more expressive, so that we can deal with qualified names.

The other map could not be removed, and has been moved instead inside `NameMangler`. The issue is that we need some way to link a typedef type to the name of the functional interface associated with the typedef. Unfortunately, despite many attempts, I realized that the typedef type does not share anything in common with the typedef declaration; as such, piggy backing on existing attributes is not feasible. So for now, I kept a map, to retain existing behavior. It is possible that the IR might have to be tweaked to accommodate this use case.

Since `NameMangler` now correctly set the functional interface name on all variables whose type is either a function pointer, or a function pointer typedef, the logic in `OutputFactory` can, again, be simplified: now `OutputFactory` only needs to fetch the name attribute set by the mangler (if any).

-------------

Commit messages:
 - undo build changes
 - Cleanup code
 - Simplify OutputFactory more
 - Simplify more code
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/151/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=151&range=00
  Stats: 651 lines in 15 files changed: 333 ins; 265 del; 53 mod
  Patch: https://git.openjdk.org/jextract/pull/151.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/151/head:pull/151

PR: https://git.openjdk.org/jextract/pull/151