[Support] Make printAllJSONValues public, for custom output.

Summary:
This changes the static method TimerGroup::printAllJSONValues from private to
public, to match the static method TimerGroup::printAll. When trying to drive
the reporting machinery by hand, the existing API is _almost_ flexible enough,
but this entrypoint is required to intermix printing timers with other
non-timer output.

The underlying motive here is a Swift change to consolidate the collection of
timers, LLVM statistics and other (non-assert-dependent) counters into JSON
files, which requires a bit of manual intervention in LLVM's stat and timer
output routines. See https://github.com/apple/swift/pull/8477 for details.

Reviewers: MatzeB

Reviewed By: MatzeB

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31566

llvm-svn: 299371