Auto merge of #101986 - WaffleLapkin:move_lint_note_to_the_bottom, r=estebank

Move lint level source explanation to the bottom

So, uhhhhh

r? `@estebank`

## User-facing change

"note: `#[warn(...)]` on by default" and such are moved to the bottom of the diagnostic:
```diff
-   = note: `#[warn(unsupported_calling_conventions)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #87678 <https://github.com/rust-lang/rust/issues/87678>
+   = note: `#[warn(unsupported_calling_conventions)]` on by default
```

Why warning is enabled is the least important thing, so it shouldn't be the first note the user reads, IMO.

## Developer-facing change

`struct_span_lint` and similar methods have a different signature.

Before: `..., impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>)`
After: `..., impl Into<DiagnosticMessage>, impl for<'a, 'b> FnOnce(&'b mut DiagnosticBuilder<'a, ()>) -> &'b mut DiagnosticBuilder<'a, ()>`

The reason for this is that `struct_span_lint` needs to edit the diagnostic _after_ `decorate` closure is called. This also makes lint code a little bit nicer in my opinion.

Another option is to use `impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>) -> DiagnosticBuilder<'a, ()>` altough I don't _really_ see reasons to do `let lint = lint.build(message)` everywhere.

## Subtle problem

By moving the message outside of the closure (that may not be called if the lint is disabled) `format!(...)` is executed earlier, possibly formatting `Ty` which may call a query that trims paths that crashes the compiler if there were no warnings...

I don't think it's that big of a deal, considering that we move from `format!(...)` to `fluent` (which is lazy by-default) anyway, however this required adding a workaround which is unfortunate.

## P.S.

I'm sorry, I do not how to make this PR smaller/easier to review. Changes to the lint API affect SO MUCH 😢

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -6,6 +6,7 @@
 //!
 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
 use hir::LangItem;
+use rustc_errors::DelayDm;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::ObligationCause;
@@ -825,13 +826,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                     DEREF_INTO_DYN_SUPERTRAIT,
                                     obligation.cause.body_id,
                                     obligation.cause.span,
-                                    |lint| {
-                                        lint.build(&format!(
-                                            "`{}` implements `Deref` with supertrait `{}` as output",
-                                            source,
-                                            deref_output_ty
-                                        )).emit();
-                                    },
+                                    DelayDm(|| format!(
+                                        "`{}` implements `Deref` with supertrait `{}` as output",
+                                        source, deref_output_ty
+                                    )),
+                                    |lint| lint,
                                 );
                                 return;
                             }