[ffigen] Remove BindingsIndex._declaredTypes

[liamappelbe]

BindingsIndex._declaredTypes is another part of the BindingsIndex that behaved inconsistently with the other parts. Most clang AST to ffigen AST transformations are cached inside the *decl_parser.dart files, but a few of them (enums, compounds, typealiases, and objC interfaces) were cached inside extractor.dart, in _declaredTypes.

I've removed _declaredTypes and refactored the caching of these entities to match the others. This significantly simplifies the extractor, since it doesn't need special handling for types that may or may not be cached. It does complicates BindingsIndex a bit, but makes it more consistent. My eventual goal is to collapse all the maps in BindingsIndex into one, and do all the caching in a single location.

[github-actions]

PR Health

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/hooks_runner/test_data/download_assets/hook/build.dart
pkgs/objective_c/example/command_line/lib/main.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

This check can be disabled by tagging the PR with skip-license-check.

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

This check can be disabled by tagging the PR with skip-leaking-check.

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

This check can be disabled by tagging the PR with skip-breaking-check.

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

This check can be disabled by tagging the PR with skip-changelog-check.

[coveralls]

coverage: 82.888% (+0.2%) from 82.722%
when pulling 3bd46bf on ffigen_cleanup_6
into 9d538ae on main.