Provide a way to build the compiler with --error-format=json

[Manishearth]

Preferably as a flag to ./x.py; preferably such that the output gets stored in the build dir.

We already run cargo with --message-format=json so we may need to differentiate somehow.

cc @Mark-Simulacrum

[Mark-Simulacrum]

If someone would like to implement this I'd be happy to write up mentoring instructions, otherwise I'll probably do it sometime soon.

@Mark-Simulacrum has been mentoring me on this - I'll submit a pull request soon!

[Mark-Simulacrum]

Opened #48558 to fix the missed fact that we hide JSON output anyway