[llvm] [IR2Vec] Refactor vocabulary to use section-based storage (PR #158376)

[S. VenkataKeerthy via llvm-commits]

svkeerthy wrote:

### Merge activity

* **Oct 2, 12:11 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/158376).


https://github.com/llvm/llvm-project/pull/158376