Don't run background-index.test on Windows.

The test had a "UNSUPPORTED: win32" line, but the spelling of that
changed in r339307 a year ago. Finally update this test too.

Part of PR43592.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@374745 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed