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

[Tom Honermann via llvm-commits]

tahonermann wrote:

I've been continuing to work on this, but have yet to get all of our internal tests to pass. I've reverted the PR to a draft pending successful internal testing.

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