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

[Richard Smith via cfe-commits]

zygoloid wrote:

This choice seems like a question of how the driver's command line is interpreted, rather than a language mode, so I wonder if it makes sense for it to consider `-fms-compatibility` at all or whether this should just be based on the driver mode. Do you have rationale for this also changing under `-fms-compatibility`?

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