Support C++ Modules in code analysis

[Gabriel Dos Reis (gdr-at-ms)]

Summary

Add support for analyzing CMake projects that use C++ Modules (named modules, module partitions, and module implementation units).

Problem

The action fails on projects with C++ Modules because it passes each source file to cl.exe /analyze:only independently, without awareness of module dependencies. This causes errors because:

• Module interface units require -interface and -ifcOutput flags
• Sources that import modules need -reference flags to resolve imports
• Module interfaces must be analyzed before their consumers

See #44 for the original report.

Fix

The fix leverages two artifacts that CMake already generates during its dependency scan phase:

1. P1689R5 dependency files (.ddi) — contain provides and requires entries for each source, enabling construction of a module dependency graph.

2. Module map files (.modmap) — contain the exact -interface, -ifcOutput, and -reference flags needed for each source's compilation.

Changes to index.js:

• CompileCommand: new ddiFile and modmapFile fields to track CMake-generated artifacts alongside each source.
• loadCompileCommands: locate .ddi and .modmap files for each source in the CMake build directory.
• resolveModuleDependencies (new function): read .ddi files to build a module dependency graph, add implicit partition-to-primary dependencies, and topologically sort sources so providers are analyzed before consumers.
• createAnalysisCommands: iterate in topological order and pass each source's @<modmap> response file to cl.exe so it receives the correct module flags.

Key design decisions:

• CMake does the heavy lifting. The action doesn't parse module semantics — it reads CMake's .ddi for ordering and passes CMake's .modmap verbatim. If CMake changes its module support, the action follows automatically.
• Zero overhead for non-module projects. If no .ddi files exist or none contain module providers, the new code paths are not taken and the original ordering is preserved.
• Partition workaround. CMake's dependency scanner may not capture the implicit dependency between a primary module interface and its partitions. The action infers this from the naming convention (X depends on X:<part>).

Testing

Tested locally against the IPR project (branch modules/cxx-ipr-traversal-io) which has 7 C++ Module interface units, 4 module implementation units, and 4 non-module source files. All sources that exist on the branch pass analysis (10 passed, 1 pre-existing input.cxx failure unrelated to modules).

Fixes#44