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

[Reid Kleckner via cfe-commits]

rnk wrote:

I would stick with `fms-compatibility` being the setting here rather than plumbing the driver mode through. Over the years, many build systems have been adapted to work with `clang[++]` and `cl` without going through clang-cl.

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