change TimerGroup to keep a linked list of active timers

change TimerGroup to keep a linked list of active timers
instead of just a count of them, and refactor the guts of
report printing out of removeTimer into its own method.
Refactor addTimerToPrint away.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99872 91177308-0d34-0410-b5e6-96231b3b80d8