[Timers] TimerGroup: make printJSONValues() method public

Summary:
This is needed for the continuation of D46504,
to be able to store the timings.

Reviewers: george.karpenkov, NoQ, alexfh, sbenza

Reviewed By: alexfh

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D46938

llvm-svn: 332505