Make I/O redirection handling in sys::Program a bit more consistent. No

functional changes. Win32 code is untested, but should work fine.

In the unix variant, rename RedirectFD to RedirectIO and let that function
handle empty and null paths instead of doing that in the caller 3 times. This
is the same as win32 already does it.

In the win32 variant, use Path::isEmpty() instead of checking the resulting
c_str() manually. This is the same as unix already does it.

llvm-svn: 52230