This PR switches to getting rustc's diagnostics message through Cargo's "JSON messages" functionality, which is described in https://doc.rust-lang.org/1.59.0/cargo/reference/external-tools.html#json-messages.

This doesn't immediately offer an improvement, but this is a necessary step to leveraging Cargo's new --keep-going flag to build all the ui test inputs simultaneously instead of one after the other, which will be an enormous speed improvement. Without JSON messages, tests which are compiled simultaneously would end up interleaving diagnostics in a way that makes it impossible to separate and attribute each diagnostic to which input file's build it relates to.

See rust-lang/cargo#10383 & rust-lang/cargo#10496 for more on --keep-going.

I confirmed that cxx and serde, both of which have large ui test suites, both pass with this change.

• https://github.com/dtolnay/cxx/tree/1.0.66/tests/ui
• https://github.com/serde-rs/serde/tree/v1.0.136/test_suite/tests/ui