Fix PathMappingListTest on windows

r334615 changed the the value of FileSpec.IsRelative("/") for windows
path syntax. We previously considered it absolute but now it is
considered relative (I guess because it's interpretation depends on the
current drive).

This cause a failure in PathMappingList test, which assumed that "/"
will not get remapped as it is an absolute path. As this is no longer
true on windows, I replace "/" with a really absolute path.

llvm-svn: 334702