[Bug driver/89026] [mingw] -I directives in response files assume C:/ as '/', but C:/msys2 when passed directly

[felipe at expertisesolutions dot com.br]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89026

--- Comment #2 from Felipe Magno de Almeida <felipe at expertisesolutions dot 
com.br> ---
Also happens when I call cc1 directly. So, it doesn't seem to be the driver's
fault.