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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM but please allow some time for @rnk or @MaskRay to review before landing

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