Escaping backslashes results in unnatural looking output, and the actual
escapes are mostly unnecessary. We were also not consistent about when
we escaped backslashes and when we didn't, making it impossible to e.g.
store the InstalledDir output to a FileCheck variable and then use that
variable to check paths relative to the driver.
This change could be generalized to any platform which uses the
backslash as a path separator instead of just Windows. I'm unaware of
other such platforms, however, and I'm also unsure if not escaping the
backslash would be appropriate for those platforms, so I'm limiting to
Windows for now.