[libcxx] [test] Fix building create_directory in MSVC configurations

Don't use the mode_t type - the official windows sdk doesn't have that type.
(Mingw headers does have such a typedef though.) The umask function returns
int on windows, in both header variants.

Thus just use auto to deduce the umask return type automatically.

Differential Revision: https://reviews.llvm.org/D98140