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

[Tom Honermann via llvm-commits]

tahonermann wrote:

@zygoloid,
> On the fourth hand, IIRC the choice of whether header search for a relative path looks in the directory of includers as well as the directory of the current file already depends on `-fms-compatibility`, and this seems sort of similar.

I confirmed the search of the include stack directories is controlled by `-fms-compatibility`. The relevant code is https://github.com/llvm/llvm-project/blob/f58ce1152703ca753794b8cef36da30bd2668d0f/clang/lib/Lex/PPDirectives.cpp#L1003-L1013.

> So yeah, I dunno what we should do; it doesn't seem clear to me.

Yup. My intuition has been to follow the existing precedent unless/until motivation to do otherwise becomes apparent.

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