[Support] Add a public API to allow clearing all (static) timer groups.

Summary:
Formerly, all timer groups were automatically cleared when printed out. In
https://reviews.llvm.org/rL324788 this behaviour was changed to not-clearing
timers on printout, to allow printing timers more than once, but as a result
clients (specifically Swift) that relied on the clear-on-print behaviour to
inhibit duplicate timer printing on shutdown were broken.

Rather than revert that change, this change adds a new API that enables
clients that _want_ to clear all timers to do so explicitly.

Reviewers: george.karpenkov, thegameg

Reviewed By: george.karpenkov

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50874

llvm-svn: 339980