[Timer] Move DefaultTimerGroup into a ManagedStatic.

This used to be just leaked. r295370 made it use magic statics. This adds
a global destructor, which is something we'd like to avoid. It also creates
a weird situation where the mutex used by TimerGroup is re-created during
global shutdown and leaked.

Using a ManagedStatic here is also subtle as it relies on the mutex
inside of ManagedStatic to be recursive. I've added a test for that
in a previous change.

llvm-svn: 304156