PrintStatistics() and PrintStatisticsJSON() should take StatLock

These two functions iterate over the list of statistics but don't take the lock
that protects the iterators from being invalidated by
StatisticInfo::addStatistic().

So far, this hasn't been an issue since (in-tree at least) these functions are
called by the StatisticInfo destructor so addStatistic() shouldn't be called
anymore. However, we do expose them in the public API.

Note that this only protects against iterator invalidation and does not protect
against ordering issues caused by statistic updates that race with
PrintStatistics()/PrintStatisticsJSON().

Thanks to Roman Tereshin for spotting it

llvm-svn: 326834