Store the hash in SectionPiece.

This spreads out computing the hash and using it in a hash table. The
speedups are:

firefox
  master 6.811232891
  patch  6.559280249 1.03841162939x faster
chromium
  master 4.369323666
  patch  4.33171853 1.00868134338x faster
chromium fast
  master 1.856679971
  patch  1.850617741 1.00327578725x faster
the gold plugin
  master 0.32917962
  patch  0.325711944 1.01064645023x faster
clang
  master 0.558015452
  patch  0.550284165 1.01404962652x faster
llvm-as
  master 0.032563515
  patch  0.032152077 1.01279662275x faster
the gold plugin fsds
  master 0.356221362
  patch  0.352772162 1.00977741549x faster
clang fsds
  master 0.635096494
  patch  0.627249229 1.01251060127x faster
llvm-as fsds
  master 0.030183188
  patch  0.029889544 1.00982430511x faster
scylla
  master 3.071448906
  patch  2.938484138 1.04524944215x faster

This seems to be because we don't stall as much. When linking firefox
stalled-cycles-frontend goes from 57.56% to 55.55%.

With -O2 the difference is even more significant since we avoid
recomputing the hash. For firefox we go from 9.990295265 to
 9.149627521 seconds (1.09x faster).

llvm-svn: 283367