[llvm] [llvm-ir2vec] Added Enum for ir2vec embedding mode (PR #190466)

[Nishant Sachdeva via llvm-commits]

nishant-sachdeva wrote:

> LG. Any reason to hardcode mode to be `symbolic` always? Why not get the mode from user?

No particular reason. I've removed the default value from the function args. 

https://github.com/llvm/llvm-project/pull/190466