Ben Bellick authored Jul 05, 2023 and Marge Bot committed Dec 24, 2023

Addresses #19278

This commit deprecates the underspecified -ddump-json flag and
introduces a newer, well-specified flag -fdiagnostics-as-json.

Also included is a JSON schema as part of the documentation.

The -ddump-json flag will be slated for removal shortly after this merge.