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

[Tom Honermann via cfe-commits]

tahonermann wrote:

This PR is finally ready for review. I would especially appreciate it if @rnk and @nico took a close look. Please add other known stakeholders.

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