[clang] [llvm] [clang] Implement JSON formatted -ftime-report (PR #137737)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Changes LGTM, but they should come with a release note in `clang/docs/ReleaseNotes.rst` so users know about the improvement, and we should also start documenting more about time reports (orthogonal to this PR).

https://github.com/llvm/llvm-project/pull/137737