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

[Tom Honermann via cfe-commits]

tahonermann wrote:

Friendly ping for additional code review. @rnk, @nico, @zmodem, @majnemer, @zygoloid.

The Windows build is failing spuriously. I've rebased and force pushed multiple times to try to get a clean build, but it keeps failing during git checkout.

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