[Support, Windows] Handle long paths with unix separators

Summary:
The function widenPath() for Windows also normalizes long path names by
iterating over the path's components and calling append().  The
assumption during the iteration that separators are not returned by the
iterator doesn't hold because the iterators do return a separator when
the path has a drive name.  Handle this case by ignoring separators
during iteration.

Reviewers: rnk

Subscribers: danalbert, srhines

Differential Revision: https://reviews.llvm.org/D36752

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311382 91177308-0d34-0410-b5e6-96231b3b80d8