MC: Pack MCSymbol::Index in with the bitfield in MCSymbol

Save a pointer for each `MCSymbol`, bringing `llc` memory usage down
from 920 MB to 914 MB, around ~0.6%.

(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)

llvm-svn: 238003