[Timers] Fix printing some `-ftime-report` sections twice. Fixes PR40328.

Starting from r324788 timer groups aren't cleared automatically when
printed out. As a result some timer groups were printed one more time.
For example, "Pass execution timing report" was printed again in
`ManagedStatic<PassTimingInfo>` destructor, "DWARF Emission" in
`ManagedStatic<Name2PairMap> NamedGroupedTimers` destructor.

Fix by clearing timer groups manually.

Reviewers: thegameg, george.karpenkov

Reviewed By: thegameg

Subscribers: aprantl, jkorous, dexonsmith, ributzka, aras-p, cfe-commits

Differential Revision: https://reviews.llvm.org/D67683

llvm-svn: 372191