Fix #71196.

These were introduced since I last fixed most of these occurences. (clippy::clone_on_copy, clippy::single_char_pattern)

remove redundant clones, references to operands, explicit boolean comparisons and filter(x).next() calls.

```
error[E0412]: cannot find type `T` in this scope
 --> file.rs:3:12
  |
3 | impl Trait<T> for Struct {}
  |     -      ^ not found in this scope
  |     |
  |     help: you might be missing a type parameter: `<T>`
```

Fix #64298.

- remove syntax::{help!, span_help!, span_note!}
- remove unused syntax::{struct_span_fatal, struct_span_err_or_warn!, span_err_or_warn!}
- lintify check_for_bindings_named_same_as_variants + conflicting_repr_hints
- inline syntax::{struct_span_warn!, diagnostic_used!}
- stringify_error_code! -> error_code! & use it more.
- find_plugin_registrar: de-fatalize an error
- de-fatalize metadata errors
- move type_error_struct! to rustc_typeck
- struct_span_err! -> rustc_errors

Point at the span for the definition of ADTs internal to the current
crate.

Look at the leading char of the ident to determine whether we're
expecting a likely fn or any of a fn, a tuple struct or a tuple variant.

Turn fn `add_typo_suggestion` into a `Resolver` method.

This is done by moving some data definitions to syntax::expand.

When detecting a possible `=` -> `:` typo in a `let` binding, suggest
assigning instead of setting the type.