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
Happy to split this up as much as makes sense. Mainly posting to show
examples for https://reviews.llvm.org/D112288.