Check for $PWD in llvm::sys::current_path.

Some users (clang, libTooling) require this. After this patch we can remove
the calls to getenv("PWD") from clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188125 91177308-0d34-0410-b5e6-96231b3b80d8