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

[Richard Smith via cfe-commits]

zygoloid wrote:

I've been pondering this. On the one hand, people switching from the `cl.exe`-compatible driver to the GCC-compatible driver might want the MSVC interpretation of the path flags. On the other hand, people enabling `-fms-compatibility` to accept code written against MSVC in a different environment might be surprised if other command-line flags behave differently (especially include paths unrelated to those files). On the third hand, this only affects cases where the same path is specified in more than one kind of list, which seems like it's probably rare. 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. So yeah, I dunno what we should do; it doesn't seem clear to me.

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