[llvm] [ObjectYAML][NFC] Hoist ContiguousBlobAccumulator into a shared header (PR #207306)

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

Couple of nits, otherwise LGTM.

The CoPilot comments that are actually valid should be done in a separate PR, to keep this NFC.

https://github.com/llvm/llvm-project/pull/207306