[STATS] Add a total statistics count
This change removes synthesized stats and instead has all timers print out a
total which is the aggregate statistics across threads. This is displayed as
"Total_foo" at the end of program. The stats_flags_e::synthesized flag is
removed and the printStats() function is split into two separate functions:
printTimerStats() which can display the aggregate total and printCounterStats().
Differential Revision: http://reviews.llvm.org/D17869