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

[Tom Honermann via cfe-commits]

tahonermann wrote:

*Sigh*, reverted to draft again. I noticed a vestigial file in both test cases, a misplaced `#include_next` directive, and on further investigation discovered that there is an additional divergence in the MSVC behavior that the patch doesn't address (it looks like a user include path that is later duplicated as a system include path is promoted to a system include path in place such that its position is preserved relative to other system include paths; but I need to investigate further).

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