Use the new sys::path::system_style() in a few places that need to detect the system's native path style, removing a few `_WIN32` macro checks. Added one FIXME to a FileManagerTest that seemed fishy, but maintained the existing behaviour. Happy to split this up as much as makes sense. Mainly posting to show examples for https://reviews.llvm.org/D112288.

Use the new sys::path::is_style_posix() and is_style_windows() in a few places that need to detect the system's native path style. In llvm/lib/Support/Path.cpp, this patch removes most uses of the private `real_style()`, but it leaves alone anything related to slash direction preferences since there's other work going on in that area. Elsewhere, this removes `_WIN32` macro checks. Added a FIXME to a FileManagerTest that seemed fishy, but maintained the existing behaviour. Happy to split this up as much as makes sense. Mainly posting to show examples for https://reviews.llvm.org/D112288.

Use the new sys::path::system_style() in a few places that need tois_style_posix() and is_style_windows() in a few places that need to detect the system's native path style. In llvm/lib/Support/Path.cpp, removing a few `_WIN32` macrothis patch removes most uses of the checks. Added one FIXME to a FileManagerTest that seemed fishyprivate `real_style()`, but it leaves alone anything related to slash maintained the existing behaviourdirection preferences since there's other work going on in that area. Elsewhere, this removes `_WIN32` macro checks. Added a FIXME to a FileManagerTest that seemed fishy, but maintained the existing behaviour. Happy to split this up as much as makes sense. Mainly posting to show examples for https://reviews.llvm.org/D112288.