Start porting ivfsoverlay tests to Windows

Part of PR43272, the changes are:

1. Use @ as the sed pattern delimiter instead of : so that the drive
letter in lit substitutions isn't an issue.

2. Use the %/t and %/S substitutions to get paths with forward slashes
to work around string quoting issues in the yaml file.

3. Replace REQUIRES:shell with XFAIL:windows. These tests should pass on
Windows, but do not for reasons that are not yet understood. We would
like to know if they pass unexpectedly.

I was able to remove the XFAILs from two tests, since they already pass
with my sed fix:
  clang/test/VFS/module_missing_vfs.m
  clang/test/VFS/test_nonmodular.c

Reviewers: amccarth

Differential Revision: https://reviews.llvm.org/D67454

llvm-svn: 371663