[llvm] [NFC] CoroElide: Refactor `Lowerer` into `CoroIdElider` (PR #91539)

[Chuanqi Xu via llvm-commits]

https://github.com/ChuanqiXu9 approved this pull request.

Took a quick look and it seems good with nits.

https://github.com/llvm/llvm-project/pull/91539