Win: handle \\?\UNC\ prefix in realPathFromHandle (PR43204)

After r361885, realPathFromHandle() ends up getting called on the working
directory on each Clang invocation. This unveiled that the code didn't work for
paths on network shares.

For example, if one maps the local dir c:\src\tmp to x:

  net use x: \\localhost\c$\tmp

and run e.g. "clang -c foo.cc" in x:\, realPathFromHandle will get
\\?\UNC\localhost\c$\src\tmp\ back from GetFinalPathNameByHandleW, and would
strip off the initial \\?\ prefix, ending up with a path that doesn't work.

This patch makes the prefix stripping a little smarter to handle this case.

Differential revision: https://reviews.llvm.org/D67166

Change-Id: Ia7c6e9955bf854a95d9184c6571f9293d467f76f
llvm-svn: 371035
(cherry picked from commit 91a5a2af)