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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@238003 91177308-0d34-0410-b5e6-96231b3b80d8

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