```
OpBuilder OpBuilder::Listener
^ ^
| |
RewriterBase RewriterBase::Listener
```
* Clients can listen to IR modifications with `RewriterBase::Listener`.
* `RewriterBase` no longer inherits from `OpBuilder::Listener`.
* Only a single listener can be registered at the moment (same as `OpBuilder`).
RFC: https://discourse.llvm.org/t/rfc-listeners-for-rewriterbase/68198
Differential Revision: https://reviews.llvm.org/D143339