[llvm] [mlir][EmitC] Add Bazel rules for conversions (PR #138154)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **May 1, 1:26 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/138154).


https://github.com/llvm/llvm-project/pull/138154