clang: Make rewrite-includes-macros.cpp runnable on non-Win

As far as I can tell, there's nothing Windows-specific about the
test and it passes fine on other platforms.

I found this test when running

    rg clang_cl clang/test | rg '%s' | rg -v -- ' -- ' | rg -v not

after 547ee1c to see if other tests were missing `--`
before `%s` in `%clang_cl` invocations. This was the only one.
Since it used to run only on Windows, it wasn't needed, but as far
as I can tell there's no reason to run it only on Windows.

Differential Revision: https://reviews.llvm.org/D158279

clang/test/Frontend/rewrite-includes-macros.cpp

@@ -1,7 +1,8 @@
-// REQUIRES: system-windows
-// RUN: %clang_cl /E -Xclang -frewrite-includes %s | %clang_cl /c -Xclang -verify /Tp -
+// RUN: %clang_cl /E -Xclang -frewrite-includes -- %s | %clang_cl /c -Xclang -verify /Tp -
 // expected-no-diagnostics
 
+// This test uses dos-style \r\n line endings.
+// Make sure your editor doesn't rewrite them to unix-style \n line endings.
 int foo();
 int bar();
 #define HELLO \