[Tests] Make OS features consistent

lit.py currently allows any substring of `target_triple` to be used as a
feature in REQUIRES/UNSUPPORTED/XFAIL. This results in various forms of
the OS spread across the tests and is also somewhat confusing since they
aren't actually listed in the available features.

Modify all OS-related features to use the `OS=` version that Swift adds
instead. We can later remove `config.target_triple` so that these don't
the non-OS versions don't work in the first place.

test/ModuleInterface/BadStdlib.swiftinterface

@@ -16,4 +16,4 @@ import ClangMod
 public func useHasPointer(_: HasPointer)
 
 // FIXME: SR-14489
-// UNSUPPORTED: windows
+// UNSUPPORTED: OS=windows-msvc