Rollup merge of #162946 - nnethercote:simplify-query-stack-printing, r=estebank

Simplify query stack printing

Currently it uses `eprintln!` for the "query stack during panic" header and "end of query stack" footer and
`struct_failure_note(..).with_span(..).emit()` for the stack frames. The difference is curious.

Looking more closely:
- failure notes lack a leading label (e.g. no "note: ");
- although the failure note is given a span, the span never renders because the `AnnotateSnippetEmitter` used for the query stack has an empty source map field;
- the JSON emitter is never involved here.

Therefore, the only difference between using `struct_failure_note` and `eprintln!` is that the former prints in bold. But boldness for frames isn't important; indeed, normal backtrace frames don't use bold.

So the commit changes query stack frame printing to use `eprintln!`, removes some now unnecessary `dcx` arguments, and also removes `struct_failure_note`. That leaves "For more information about an error, try `rustc --explain {}`" as the only use of `FailureNote`.

r? @estebank