FileSystem: Check for DTTOIF alone, not _DIRENT_HAVE_D_TYPE

While 'd_type' is a non-standard extension to `struct dirent`, only
glibc signals its presence with a macro '_DIRENT_HAVE_D_TYPE'.
However, any platform with 'd_type' also includes a way to convert to
mode_t values using the macro 'DTTOIF', so we can check for that alone
and still be confident that the 'd_type' member exists.

(If this turns out to be wrong, I'll go back and set up an actual
CMake check.)

I couldn't think of how to write a test for this, because I couldn't
think of how to test that a 'stat' call doesn't happen without
controlling the filesystem or intercepting 'stat', and there's no good
cross-platform way to do that that I know of.

Follow-up (almost a year later) to r342089.

rdar://problem/50592673
https://reviews.llvm.org/D64940

llvm-svn: 366486