Add GetCurrentDirectory back.

It looks like clang-tools-extra/unittests/cpp11-migrate/TransformTest.cpp
depends on the behaviour of the old one on Windows. Maybe a difference
between GetCurrentDirectoryA and GetCurrentDirectoryW?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184009 91177308-0d34-0410-b5e6-96231b3b80d8