Expose three helpers in namespace llvm::sys::path to detect the
path rules followed by sys::path::Style.
- is_style_posix()
- is_style_windows()
- is_style_native()
This are constexpr functions that that will allow a bunch of
path-related code to stop checking _WIN32.
Originally I looked at adding system_style(), analogous to
sys::endian::system_endianness(), but future patches (from others) will
add more Windows style variants for slash preferences. These helpers
should be resilient to that change, allowing callers to detect basic
path rules.