[lit] Fix running gtest type-parameterized tests on Windows

The '/' character in the test name of a type-parameterized test is not a
path separator, and should not be '\' on Windows. We were passing a test
name to --gtest_filter which found no tests, so the exit code was zero,
indicating a passed test.

This bug has been here since r84387 in 2009, when Jeff Yasskin added the
original lit support for type-paratermized tests. Somewhere along the
line some of the ValueMapTests started failing, but we can fix those
separately.

llvm-svn: 234242