[OpenMP][Stats] Cleanup stats gathering code

1) Remove unnecessary data from list node structure
2) Remove timerPair in favor of pushing/popping explicitTimers.
   This way, nested timers will work properly.
3) Fix #pragma omp critical timers
4) Add histogram capability
5) Add KMP_STATS_FILE formatting capability
6) Have time partitioned into serial & parallel by introducing
   partitionedTimers::exchange(). This also counts the number of serial regions
   in the executable.
7) Fix up the timers around OMP loops so that scheduling overhead and work are
   both counted correctly.
8) Fix up the iterations statistics so they count the number of iterations the
   thread receives at each loop scheduling event
9) Change timers so there is only one RDTSC read per event change
10) Fix up the outdated comments for the timers

Differential Revision: https://reviews.llvm.org/D49699

llvm-svn: 338276