[clang] [Clang] Match MSVC handling of duplicate header search paths in Microsoft compatibility modes. (PR #105738)

[Tom Honermann via cfe-commits]

tahonermann wrote:

It took me a few tries to get the tests tweaked just right for the `clang-cl` driver to pass on Windows, but the tests now pass and this PR is now ready for review.

https://github.com/llvm/llvm-project/pull/105738