Preserve generated check segments

[1sgtpepper]

Summary

Keep generated FileCheck attribute-definition checks in the source segment that produced them, so split-input output preserves source order. This focused prerequisite PR partially addresses the test-generator work needed by #484; it does not change fusion behavior.

Related work

Partially addresses #484, following the maintainer-requested split.

Changes

• Append each generated attribute-definition check to the current output segment.
• Add a two-segment regression that runs the generator with --source and verifies both definitions remain with their producing segment.
• Document the segment-order contract in the generator usage text and changelog.
• Keep the single-segment path unchanged when --source is not used.

Testing

All required hosted CI checks pass for head ed20057f95b0dc43c91c7f1648863f3bd3ba40fb on the checks. The new regression exercises the generator and FileCheck together; the focused generator-order harness and Python compilation also pass.

Submission checklist

• If I am an external contributor, this PR has a linked issue marked approved; otherwise, this does not apply.
• I added or updated tests for all relevant behavior, or explained above why tests are not needed.
• I updated the relevant TableGen or other documentation, or explained above why documentation is not needed.
• I added a changelog entry describing user-visible changes.
• I enabled Allow edits from maintainers.

AI assistance

• No AI tools contributed to this PR.
• AI tools contributed to this PR.

Tools used: Codex

How the tools contributed: Implementation and code review.

How I verified the contribution: Final changes reviewed.

[project-llzk-issue-triage]

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Closes #123

[tim-hoffman]

LGTM