Rollup merge of #100730 - CleanCut:diagnostics-rustc_monomorphize, r=davidtwco

Migrate rustc_monomorphize to use SessionDiagnostic

### Description

- Migrates diagnostics in `rustc_monomorphize` to use `SessionDiagnostic`
- Adds an `impl IntoDiagnosticArg for PathBuf`

### TODO / Help!
- [x] I'm having trouble figuring out how to apply an optional note. 😕  Help!?
  - Resolved. It was bad docs. Fixed in https://github.com/rust-lang/rustc-dev-guide/pull/1437/files
- [x] `errors:RecursionLimit` should be `#[fatal ...]`, but that doesn't exist so it's `#[error ...]` at the moment.
  - Maybe I can switch after this is merged in? --> https://github.com/rust-lang/rust/pull/100694
  - Or maybe I need to manually implement `SessionDiagnostic` instead of deriving it?
- [x] How does one go about converting an error inside of [a call to struct_span_lint_hir](8064a49508/compiler/rustc_monomorphize/src/collector.rs (L917-L927))?
- [x] ~What placeholder do you use in the fluent template to refer to the value in a vector? It seems like [this code](0b79f758c9/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs (L83-L114)) ought to have the answer (or something near it)...but I can't figure it out.~ You can't. Punted.

compiler/rustc_typeck/src/coherence/builtin.rs

@@ -359,7 +359,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUn
     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 
     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
-        tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
+        tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
     });
 
     let source = tcx.type_of(impl_did);