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(), where is_style_posix() and is_style_windows()
are just a little tidier.
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.