Add help text for -MMD, -MD, -MM, -M, -MF.

Also regroup these flags so that alike flags are next to each other, while
keeping the list still mostly alphabetical.

The help text isn't ideal, but I feel it's less maze-like than
http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Preprocessor-Options.html (look
at the entry for '-MMD' and count how many other entries you need to look up
until you know what it does).

And it looks like -M / -MM are mostly an historical accident and most people
use -MD or -MMD for deps tracking these days, so make -M / -MM refer to
-MD / -MMD instead of the other way round.

llvm-svn: 199067