Add parallel-frontend-threads to bootstrap.toml and enable multi-thr…

[Hayden602]

…eaded parallel compilation

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR modifies bootstrap.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

r? @Mark-Simulacrum

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[Hayden602]

r? @SparrowLii Hi，I’ve finished handling the issue regarding enabling parallel front end in bootstrap. Please help review if it looks appropriate.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.