[MLIR][NFC] Get DiagnosticEngine as a reference in doc

[gAlfonso-bit]

Summary

'mlir::DiagnosticEngine::DiagnosticEngine(const mlir::DiagnosticEngine&)' is implicitly deleted because the default definition would be ill-formed.

[rdzhabarov]

Nit:

[MLIR] get DiagnosticEngine as a reference

could you add [NFC] as well.

[gAlfonso-bit]

Done! Maybe we can also cherry pick this to release 13 too since its a NFC change?

[gAlfonso-bit]

Any updates? @rdzhabarov

[rdzhabarov]

In D107287#2925507, @gAlfonso-bit wrote:

Any updates? @rdzhabarov

Looks like I need to get your "First Last <[e-mail]>" in order to get this diff merged.
I'm not sure about the release aspect you are asking.

[gAlfonso-bit]

This comment was removed by gAlfonso-bit.

[rdzhabarov]

In D107287#2926374, @gAlfonso-bit wrote:

release 13 I mean (since this is just a doc update), and it is "Alfsonso Gregory <[e-mail]>" @rdzhabarov

Merged, but I'm not sure about the policies around releases. @mehdi_amini

mlir/docs/Diagnostics.md

 manages the registration of diagnostic handlers, as well as the core API for
 diagnostic emission. Handlers generally take the form of
 `LogicalResult(Diagnostic &)`. If the result is `success`, it signals that the
 diagnostic has been fully processed and consumed. If `failure`, it signals that
 the diagnostic should be propagated to any previously registered handlers. It
 can be interfaced with via an `MLIRContext` instance.
 
 ```c++
-DiagnosticEngine engine = ctx->getDiagEngine();
+DiagnosticEngine& engine = ctx->getDiagEngine();
 
 /// Handle the reported diagnostic.
 // Return success to signal that the diagnostic has either been fully processed,
 // or failure if the diagnostic should be propagated to the previous handlers.
 DiagnosticEngine::HandlerID id = engine.registerHandler(
     [](Diagnostic &diag) -> LogicalResult {
   bool should_propagate_diagnostic = ...;
   return failure(should_propagate_diagnostic);