`x.py --json-output` suppresses `CompilerArtifact` messages

[RalfJung]

When using x.py --json-output, I would expect JSON output to be basically the same as standard cargo. However, that does not seem to be the case: an analysis of rust-lang/rust-analyzer#4827 by @bjorn3 revealed that CompilerArtifact are not propagated properly.

Cc @Mark-Simulacrum

[Mark-Simulacrum]

Problem is here: https://github.com/rust-lang/rust/blob/master/src/bootstrap/compile.rs#L985-L989

Those lines should probably be modified such that we unconditionally print the received line if the --json-output flag is passed?

[bjorn3]

Yes