test: disable Index.Store.output-failure on Windows

`chmod -w` really doesn't make sense on Windows.  Although NTFS allows
for readonly directories, it is something done through file system level
ACLs.  This test doesn't really apply to Windows.

Author: compnerd

test/Index/Store/output-failure.swift

@@ -1,5 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: mkdir %t/idx
+// UNSUPPORTED: OS=windows-msvc
 
 // Before indexing, do a dry-run to ensure any clang modules are cached. We
 // want to isolate the error that comes from swift's indexing support, not