[Timers] TimerGroup::printJSONValues(): print mem timer with .mem suffix

Summary: We have just used `.sys` suffix for the previous timer, this is clearly a typo

Reviewers: george.karpenkov, NoQ, alexfh, sbenza

Reviewed By: alexfh

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D46936

llvm-svn: 332503