[tests] Add edge case coverage for CopyIncludePaths in Paths.cpp

[voyager-jhk]

Description

This PR improves the test coverage for the internal utility function CopyIncludePaths located in lib/CppInterOp/Paths.cpp.

Previously, several edge-case flag branches (such as -fmodule-cache-path, -nostdinc, -stdlib=libc++, and -v) lacked test coverage.

I have:

1. Created a dedicated PathsTest.cpp test suite.
2. Added PathsTest.CopyIncludePathsEdgeCases to specifically target and validate these uncovered branches.
3. Updated the unittests/CppInterOp/CMakeLists.txt to properly link the new test file.

Contributes to #541

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Requires documentation updates
• Test coverage improvement

Testing

Please describe the test(s) that you added and ran to verify your changes.

• Built the CppInterOpTests target locally using CMake.
• Successfully ran the new test suite via Google Test filter: ./unittests/CppInterOp/bin/Release/CppInterOpTests --gtest_filter="PathsTest.*"
• Verified that all edge-case branches are correctly hit and pass without assertions or memory errors.

Checklist

• I have read the contribution guide recently

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.90%. Comparing base (22812bc) to head (673ed62).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #862      +/-   ##
==========================================
+ Coverage   79.75%   79.90%   +0.14%     
==========================================
  Files          11       11              
  Lines        4031     4031              
==========================================
+ Hits         3215     3221       +6     
+ Misses        816      810       -6     

see 1 file with indirect coverage changes
see 1 file with indirect coverage changes

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[github-actions]

clang-tidy made some suggestions

[voyager-jhk]

@mcbarton @aaronj0 Looks like the linker errors only happen on cling native builds and Emscripten (WASM DCE). All clang-repl native builds are green.

Since CopyIncludePaths seems unavailable in those environments, should I just guard this test in CMake/macros, or rewrite it to hit those branches indirectly via the public Interpreter API?

[github-actions]

clang-tidy made some suggestions

[mcbarton]

@voyager-jhk to fix the Emscripten linker errors like the one in the ci, you need to work out what make use of the cppinterop_api annotation. It should be obvious how to use it looking at the source code.

[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: unknown type name 'CPPINTEROP_API' [clang-diagnostic-error]

CPPINTEROP_API void CopyIncludePaths(const clang::HeaderSearchOptions& Opts,
^

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"