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

[Tom Honermann via cfe-commits]

tahonermann wrote:

> Doesn't that impact whether a header is found via `""` or `<>` syntax? e.g., if something went from a user header to a system header, I thought that meant the search order would then change depending on which include syntax you used?

No, those are orthogonal concerns, but there is unfortunate terminology overlap. Options like `-isystem` nominate system include paths and headers found within those paths are treated as system headers. However, a header found via another include path can still be considered a system header. Consider a header file that contains `#pragma GCC system_header`; such a header file is considered a system header, but doesn't influence how header file inclusion is resolved.

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