Refactor Timer class

Summary:
While removing TimeValue from this class I noticed a lot of room for small
simplifications here. Main are:
  - instead of complicated start-stop dances to compute own time, each Timer
    just starts the timer once, and keeps track of the durations of child
    timers. Then the own time can be computed at the end by subtracting the two
    values.
  - remove double accounting in TimerStack - the stack object already knows the
    number of timers.
The interface does not lend itself well to unit testing, but I have added a
couple of tests which can (and did) catch any obvious errors.

Reviewers: tberghammer, clayborg

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D26243

llvm-svn: 285890