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

[Tom Honermann via cfe-commits]

tahonermann wrote:

Thanks, @AaronBallman,

> > Ideally, I think we would do the following at some point to improve compatibility with MSVC.
> 
> I'm not opposed, but I am concerned about the potential to subtly break user code that's relying on our current search path behavior. We may need to find some clever diagnostics for cases where lookup would have previously succeeded or found a different file.

The changes I was suggesting in that comment would not change what files are found; it would only affect whether the file was to be treated as a user or system header. That being said, the changes in this PR do affect file resolution, but only in cases where the same path is specified multiple times.

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