[XRay][tools] Use Support/JSON.h in llvm-xray convert

Summary:
This change removes the ad-hoc implementation used by llvm-xray's
`convert` subcommand to generate JSON encoded catapult (AKA Chrome
Trace Viewer) trace output, to instead use the JSON encoder now in the
Support library.

Reviewers: kpw, zturner, eizan

Reviewed By: kpw

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50129

llvm-svn: 338834