[llvm] [LLVM][Docs] Define IR in the lexicon (PR #221388)

[Prajit Rahul via llvm-commits]

Prajit-Rahul wrote:

Ping. @kazutakahirata, could you please take a look when you have a chance? If someone else would be a better reviewer, please let me know.

https://github.com/llvm/llvm-project/pull/221388