Driver: Add a --working-directory option which can be used to cause the compiler

to operate "as if" in a certain working directory.
 - For now, we just implement this by changing the actual working directory, but
   eventually we would want to handle this transparently. This is useful to
   avoid an extra exec() pair in some situations, and will be something we would
   want to support for more flexibility in using the Clang libraries.

llvm-svn: 140409