[libFuzzer] Implement DirName() for Windows.

Implement DirName from scratch to avoid dependencies on external libraries.
It's based on MSDN documentation for Naming Files, Paths, and Namespaces.

The algorithm can't simply start from the end and look backwards for the
first separator, because we need to preserve the prefix that represent
the root location. We shouldn't remove anything there. In Windows we
have many different options, like:
 \\Server\Share\ , \ , C: , C:\ , \\?\C:\ , \\?\UNC\Server\Share\
We remove the last separator in the rest of the path, if it exists.

It was implemented to have a similar behaviour to dirname() in linux,
removing trailing separators, returning "." when the path doesn't
contain separators, etc.

Differential Revision: https://reviews.llvm.org/D27579

llvm-svn: 289562