[ICF] Include section contents in section hash values.

Computing section content hashes early seems like a win in terms of
performance. It increases a chance that two different sections will get
different class IDs from the beginning.

Without threads, this patch improves Chromium link time by about 0.3
seconds. With threads, by 0.1 seconds. That's less than 1% time saving
but not bad for a small patch.

llvm-svn: 314644