[llvm] [IR2Vec][llvm-ir2vec] Changing clEnumValN to cl::SubCommand (PR #151384)

[S. VenkataKeerthy via llvm-commits]

svkeerthy wrote:

### Merge activity

* **Aug 2, 8:43 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/151384).


https://github.com/llvm/llvm-project/pull/151384