Add `--json=timings` option to print build timings

[Kobzol]

Proposal

I would like to add new a option to the --json compiler flag, called e.g. timings, which would print a JSON message with a very high-level overview of the time spent in individual sections of the compilation (e.g. frontend/backend/debuginfo generation/linking). This information could be then processed e.g. by Cargo and displayed in cargo build --timings (rust-lang/cargo#14265, rust-lang/cargo#10643), to help Rust users more easily diagnose where is the bottleneck in their compilation pipeline.

In terms of implementation, I'm imagining a very slimmed down version of -Zself-profile that is very cheap. Essentially just wrap the frontend/backend/linker sections of the compiler and output their wall-time duration.

Mentors or Reviewers

@Mark-Simulacrum

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[rustbot]

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[davidtwco]

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