Use NtCreateFile on Win32 (dotnet/corefx#27195)

Use NtCreateFile on Win32 (dotnet/corefx#27195)

NtCreateFile allows passing in a base handle which makes opening child directories much faster. This cuts anywhere from 3-10% of the time off of a GetFiles() call.

It is significantly faster as it avoids normalization and path parsing overhead.

Commit migrated from https://github.com/dotnet/corefx/commit/a28a2cdda67c024126f690bbe1331174a194f9ce