Support: Use sys::path::is_style_{posix,windows}() in a few places

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
behaviour.

Differential Revision: https://reviews.llvm.org/D112289