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

[Tom Honermann via cfe-commits]

tahonermann wrote:

Further internal testing has indicated that there will likely be backward compatibility problems in practice with the implemented approach of differentiating behavior based on `-fms-compatibility`. I'm therefore going to look into adding a new `-cc1` option, `-fms-header-search`, to control this behavior. The new option will only be enabled by default for `clang-cl`.

Opinions on whether the current `-fms-compatibility` controlled behavior to search the directories of the include stack for `#include "xxx"` headers should be switched to this new option is welcome. I tend to think it should be.

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