[VFS] More consistent support for Windows

[VFS] More consistent support for Windows

Removed some #ifdefs specific to Windows handling of VFS paths.  This
eliminates most of the differences between the Windows and non-Windows
code paths.

Making this work required some changes to account for the fact that VFS
file paths can be Posix style or Windows style, so you cannot just assume
that they use the host's native path style.  In one case, this means
implementing our own version of make_absolute, since the filesystem code
in Support doesn't have styles in the sense that the path code does.

Differential Review: https://reviews.llvm.org/D71092