Rollup merge of #117064 - dtolnay:handleerrors, r=cjgillot

Eliminate rustc_attrs::builtin::handle_errors in favor of emitting errors directly

Suggested in https://github.com/rust-lang/rust/pull/116773#pullrequestreview-1691411257.

This `handle_errors` function is originally from https://github.com/rust-lang/rust/pull/34531, in which it was useful because it allowed error messages and error codes (`E0542`) for multiple occurrences of the same error to be centralized in one place. For example rather than repeating this diagnostic in 2 places:

```rust
span_err!(diagnostic, attr.span, E0542, "missing 'since'");
```

one could repeat this instead:

```rust
handle_errors(diagnostic, attr.span, AttrError::MissingSince);
```

ensuring that all "missing 'since'" errors always remained consistent in message and error code.

Over time as error messages and error codes got factored to fluent diagnostics (https://github.com/rust-lang/rust/pull/100836), this rationale no longer applies. The new code has the same benefit while being less verbose (+73, -128).

```rust
sess.emit_err(session_diagnostics::MissingSince { span: attr.span });
```

r? `@cjgillot`

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