feat: Ordered search paths for `-I`, `-isystem`, `-F`, `-iframework` + Darwin framework support (with legacy `-I` back-compat) by jcfr · Pull Request #511 · cppcheck-opensource/simplecpp

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This change teaches simplecpp to resolve headers from Apple-style Framework
directories while preserving the left-to-right order of interleaved
-I/-F/-iframework search paths (like GCC/Clang on Darwin).

This enables both:
- `__has_include(<Pkg/Hdr.h>)` -> `<Pkg.framework/Headers/Hdr.h>` (or `PrivateHeaders`)
- `#include <Pkg/Hdr.h>`       -> same framework layout when a package prefix exists

Changes:
- Add `DUI::SearchPath` with `PathKind {Include, Framework, SystemFramework}`.
- If `DUI::searchPaths` is non-empty, use it verbatim (interleaved -I/-F/-iframework).
  Otherwise preserve back-compat by mirroring `includePaths` as Include paths.
- Update `openHeader()` to consult typed paths, and only rewrite `<Pkg/Hdr.h>`
  to `Pkg.framework/{Headers,PrivateHeaders}/Hdr.h` when a package prefix exists.
- Implement `toAppleFrameworkRelatives()` returning prioritized candidates
  (Headers first, then PrivateHeaders).
- Tests use `PathKind::Framework` when checking framework layout.

CLI
- Support -F<dir> and -iframework<dir> (keep -I as before).

Behavior notes
- The order of -I/-F/-iframework is preserved exactly as provided.
- `Framework` vs `SystemFramework` differ only in diagnostic semantics (not lookup).
- Legacy users who only set `DUI::includePaths` see identical behavior.

Tests
- Add `appleFrameworkHasIncludeTest` for `__has_include` resolution.
- Add `appleFrameworkIncludeTest` for `#include` resolution.
- Add dummy fixture: `testsuite/Foundation.framework/Headers/Foundation.h`.

This brings simplecpp closer to GCC/Clang behavior on macOS and enables
robust resolution of framework headers like `Foundation/Foundation.h`.

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Co-authored-by: Hans Johnson <hans-johnson@uiowa.edu>
Suggested-by: glankk <glankk@users.noreply.github.com>

This change introduces support for the -isystem flag, allowing users to
specify system include directories. The addition includes handling the
new flag in both the argument parsing and include resolution logic,
as well as updating relevant tests to validate the new functionality.