Revert "[Driver] Use forward slashes in most linker arguments"

This reverts commit r345004, as it broke tests when actually run
on windows; see e.g.
http://lab.llvm.org:8011/builders/clang-x64-windows-msvc/builds/763.

This broke tests that had captured a variable containing a path
with backslashes, which failed to match cases in the output
where the path separators had been changed into forward slashes.

llvm-svn: 345005