Change Termination::report return type to ExitCode

[yaahc]

Related to #43301

The goal of this change is to minimize the forward compatibility risks in stabilizing Termination. By using the opaque type ExitCode instead of an i32 we leave room for us to evolve the API over time to provide what cross-platform consistency we can / minimize footguns when working with exit codes, where as stabilizing on i32 would limit what changes we could make in the future in how we represent and construct exit codes.

[rust-highfive]

r? @Mark-Simulacrum

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[yaahc]

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)
Click to see the possible cause of the failure (guessed by this bot)

These test failures are unrelated to the changes in this PR: https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/Spurious.20mir-opt.20segfaults.20GHA/near/269699225

[Mark-Simulacrum]

I will note that without even a PartialEq or is_success/is_failure methods, this makes the ExitCode basically useless outside unstable code modulo passing up to fn main. That seems plausibly OK, but fairly weird for an API we would actually want to stabilize.

r=me with the one nit comment fixed and/or perf run to confirm no effects modulo it.

@bors rollup=never since regardless this'll probably perturb codegen for binaries a little.

[yaahc]

Awesome, resolved the inlining issue. Going to add the partial_eq / is_success question to the ExitCode tracking issue as an unresolved question for now.

@bors r=Mark-Simulacrum rollup=never

[bors]

📌 Commit 19db85d has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 19db85d with merge 2681f25...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 2681f25 to master...

[rust-timer]

Finished benchmarking commit (2681f25): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression