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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> 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.

My intuition is that it should be switched to the new option; we want `-fms-compatibility` to mean "be compatible with MSVC" and that shouldn't require additional flags except in exceptional cases. This doesn't feel like an exceptional case to me.

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