[driver] Revert r180652 and 180658 and temporarily #define MAXPATHLEN to
make the gdb tests and the Windows bots happy.

The Path::GetCurrentDirectory API is not equivalent to ::getcwd(), so
r180652 causes a gdb tests to fail.  On the other hand, <sys/param.h>
isn't defined on Windows systems, so that causes Windows builds to fail.
rdar://12237559