Fix handling of "clang c:foo"

On windows, c:foo is a valid file path, but stat fails on just "c:". This
causes a problem for clang since its file manager wants to cache data about
the parent directory.

There are refactorings to be done in here, but this gives clang the correct
behavior and testing first.

Patch by Yunzhong Gao!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187359 91177308-0d34-0410-b5e6-96231b3b80d8