Small improvement to the use of GetFileType:

* assert that the return value is one of the documented values on msdn.
* on FILE_TYPE_UNKNOWN, check GetLastError.

Unfortunately I can't think of a way to get a FILE_TYPE_UNKNOWN on a test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186595 91177308-0d34-0410-b5e6-96231b3b80d8