[lit] Mark several of lit's tests XFAIL on Windows

Summary:
rL257221 attempted to run lit's own test suite continuously, but that
commit was reverted because lit's test suite does not pass on Windows.
Because lit's tests do not run continuously, they often regress.

In order to un-revert rL257221, mark lit tests that fail as XFAIL for
Windows platforms.

Test Plan:
On a Windows development environment, follow the instructions in
utils/lit/README.txt to run lit's test suite:

```
utils/lit/lit.py \
    --path /path/to/your/llvm/build/bin \
    utils/lit/tests
```

Verify that the test suite is run and a successful exit code is
returned.

Reviewers: mgorny, rnk, delcypher, beanz

Reviewed By: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35879

llvm-svn: 309123