Support Using Clang Module Map to Avoid mixing #include and import pr…

[PikachuHyA]

…oblems

Motivation

We are refactoring our legacy code to enable C++20 Modules. With this approach, we do not need to change the actual business code, or only need to make very small changes. We only need to add a .modulemap file to get the benefits of C++20 Modules.

Summary

Adds a new module_header_maps attribute to cc_library and cc_binary so targets can use Clang header module maps (.modulemap + -fmodule-map-file) alongside C++20 named modules (module_interfaces).

This addresses the common case where legacy #include and import must coexist in the same translation unit: Clang can treat headers as part of a named module when a module map is provided, without requiring every header to be rewritten as a module interface. see https://clang.llvm.org/docs/StandardCPlusPlusModules.html#using-clang-module-map-to-avoid-mixing-include-and-import-problems

New attribute

• module_header_maps: dict mapping module name → .modulemap label (via string_keyed_label_dict)
• Requires --experimental_cpp_modules and the toolchain cpp_modules feature (same as module_interfaces)

Compile behavior

When module_header_maps is set on a target:

1. Per-source DDI scanning and per-source generate-modmap are skipped for .cc files on that target.
2. A single target-level modmap is produced by reusing the existing generate-modmap / CppGenModmap pipeline:
   • Synthetic DDI JSON lists all module names from the dict.
   • One shared modmap + modmap.input is generated for all .cc sources.
3. .modulemap files are passed as compile additional_compilation_inputs and emitted via a new toolchain feature cpp_module_header_map_files as -fmodule-map-file=... (Linux Clang toolchain only).

Intentional v1 limitations

To simplify the implementation

• module_header_maps is not propagated through CcCompilationContext; each target that compiles .cc and needs header maps must declare them explicitly (same pattern as early module_interfaces usage).
• No per-.cc dependency analysis: the shared modmap includes all modules listed in the dict for every .cc on the target.
• cpp_module_header_map_files is registered only on the Unix/Linux toolchain config (not Windows/macOS yet).