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

Refactor llvm-ir2vec to use subcommands instead of a mode flag for better CLI usability.

- Converted the `--mode` flag to three distinct subcommands: `triplets`, `entities`, and `embeddings`
- Updated documentation, tests, and python script

GitOrigin-RevId: 21f1f9558df3830ffa637def364e3c0cb0dbb3c0
7 files changed