Rollup merge of #128798 - futile:refactor/mbe-diagnostics, r=petrochenkov

refactor(rustc_expand::mbe): Don't require full ExtCtxt when not necessary

Refactor `mbe::diagnostics::failed_to_match_macro()` to not require a full `ExtCtxt`, but only a `&ParseSess`. It hard-required the `ExtCtxt` only for a call to `cx.trace_macros_diag()`, which we move instead to the only call-site of the function.

Note: This could be a potential change in observed behavior, because a call to `cx.trace_macros_diag()` now always happens after `failed_to_match_macro()` was called, where before it was only called at the end of the main return path of the function. But since `trace_macros_diag()` "flushes" out any not-yet-reported errors, it should be ok to call it for all paths, since there shouldn't be any on the non-main paths I think. However, I don't know the rest of the codebase well enough to say that with 100% confidence, but `tests/ui` still pass, which gives at least some confidence in the change.

Also concretize the return type from `Box<dyn MacResult>` to `(Span, ErrorGuaranteed)`, because this function will _always_ return an error, and never any other kind of result.

Was part of #128605 and #128747, but is a standalone refactoring.

r? ``@petrochenkov``

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