[WIP] metadata: Split ModChild into two structures

[petrochenkov]

to avoid encoding redundant data.

Implement the suggestion from #91795 (comment).
r? @cjgillot

[petrochenkov]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 530e76d with merge 15b7cbc9690be475512dc97619c8d4f30ff3c54b...

[bors]

☀️ Try build successful - checks-actions
Build commit: 15b7cbc9690be475512dc97619c8d4f30ff3c54b (15b7cbc9690be475512dc97619c8d4f30ff3c54b)

[rust-timer]

Queued 15b7cbc9690be475512dc97619c8d4f30ff3c54b with parent f58d51b, future comparison URL.

[rust-timer]

Finished benchmarking commit (15b7cbc9690be475512dc97619c8d4f30ff3c54b): comparison url.

Summary: This benchmark run did not return any relevant results. 18 results were found to be statistically significant but too small to be relevant.

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

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

[petrochenkov]

Not an improvement.