[Timers] TimerGroup::printJSONValue(): print doubles with no precision loss

Summary:
Although this is not stricly required, i would very much prefer
not to have known random precision losses along the way.

Reviewers: george.karpenkov, NoQ, alexfh, sbenza

Reviewed By: george.karpenkov

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D46937

llvm-svn: 332504